Deactivating virtual devices and rolling backup

ABSTRACT

Handling data includes providing a first storage area of a first type that contains sections of data, providing a second storage area of the first type that contains sections of data, providing a third storage area of a second type where the second type has, for each section thereof, a pointer to one of: a corresponding section of data of the first storage area and a corresponding section of data of the second storage area, causing the third storage area to be not available for accessing, and after causing the third storage area to not be available for accessing, providing data from the second storage area corresponding to pointers of the third storage area that point to sections of the second storage area.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/120,016 filed on Apr. 10, 2002 (pending).

BACKGROUND OF THE INVENTION

1. Technical Field

This application relates to computer storage devices, and moreparticularly to the field of providing copies of portions of data storedon a computer storage device.

2. Description of Related Art

Host processor systems may store and retrieve data using a storagedevice containing a plurality of host interface units (host adapters),disk drives, and disk interface units (disk adapters). Such storagedevices are provided, for example, by EMC Corporation of Hopkinton,Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al.,5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky etal., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access thestorage device through a plurality of channels provided therewith. Hostsystems provide data and access control information through the channelsof the storage device and the storage device provides data to the hostsystems also through the channels. The host systems do not address thedisk drives of the storage device directly, but rather, access whatappears to the host systems as a plurality of logical volumes. Thelogical volumes may or may nor correspond to the actual disk drives.

In some instances, it may desirable to provide a copy of a logicalvolume where the copy is then accessed by other processes. For example,to test new software on actual stored data, a copy of a logical volumecontaining the data may be made and the copy, as opposed to the originaldata, may be used to test new software. Once the test is complete, thecopy may be eliminated. Thus, the new software is tested on actual datawithout affecting the actual data. This reduces the likelihood thattesting new software and/or functionality will corrupt actual data.

One difficulty with making such copies is that they require as muchstorage space as the logical volume from which the data is obtainedsince the copy process simply creates a new volume containing all thedata of the original volume. In addition, in some instances, thedifferences between the original volume and the copy are minimal. Thus,the extra storage space required for such a copy of a logical volume isused somewhat inefficiently since it merely duplicates already-existingdata. Accordingly, it would be desirable to provide a mechanism forcopying data in a way that uses storage space efficiently.

SUMMARY OF THE INVENTION

According to the present invention, accessing stored data includesproviding a virtual storage area having a table of pointers that pointto sections of at least two other storage areas, where the virtualstorage area contains no sections of data, in response to a request foraccessing data of the virtual storage area, determining which particularone of the other storage areas contain the data, and accessing the dataon the particular one of the other storage areas using the table ofpointers. Accessing stored data may also include associating a first oneof the other storage areas with the virtual storage area, where thevirtual storage area represents a copy of data of the first one of theother storage areas. Accessing stored data may also include causing allof the pointers of the table to initially point to sections of the firstone of the other storage areas when the virtual storage area isinitially associated with the first one of the other storage areas.Accessing stored data may also include, in response to a write to afirst section on the first one of the other storage areas, copying dataof the first section to a second section that is on a second one of theother storage areas and causing a corresponding one of the pointers ofthe table to point to the second section. Prior to copying data from thefirst section to the second section, the second section may bemaintained as a free section containing no data. Accessing stored datamay also include maintaining a doubly linked list of all free sectionsof the second one of the other storage areas. Accessing stored data mayalso include associating a data indicator with sections of the first oneof the other storage areas, where the data indicator for a particularsection indicates whether a write operation has been performed to theparticular section after the first one of the other storage areas hasbeen associated with the virtual storage area. Accessing stored data mayalso include, in response to a write to a first section on the first oneof the other storage areas and the data indicator indicating that nowrite operation has been performed to the first section after the firstone of the other storage areas has been associated with the virtualstorage area, copying data of the first section to a second section thatis on a second one of the other storage areas and causing acorresponding one of the pointers of the table to point to the secondsection. Accessing stored data may also include sending statusinformation to a device that caused the write operation to be performedfollowing copying the data from the first section to the second section.Each of the sections of data may be a track of data. Each of the storageareas may be a storage device.

According further to the present invention, accessing stored dataincludes providing a first virtual storage area having a first table ofpointers that point to sections of at least two other storage areas,where the first virtual storage area contains no sections of data,associating a first one of the other storage areas with the firstvirtual storage area, where the first virtual storage area represents acopy of data of the first one of the other storage areas at a firstpoint in time, providing a second virtual storage area having a secondtable of pointers that point to sections of at least two other storageareas, where the second virtual storage area contains no sections ofdata, associating the first one of the other storage areas with thesecond virtual storage area, where the first virtual storage arearepresents a copy of data of the first one of the other storage areas ata second point in time, in response to a request for accessing data ofone of the virtual storage areas, determining which particular one ofthe other storage areas contain the data, and accessing the data on theparticular one of the other storage areas using one of the tables ofpointers. Accessing stored data may also include causing all of thepointers of the first table to initially point to sections of the firstone of the other storage areas when the first virtual storage area isinitially associated with the first one of the other storage areas andcausing all of the pointers of the second table to initially point tosections of the first one of the other storage areas when the secondvirtual storage area is initially associated with the first one of theother storage areas. Accessing stored data may further includeassociating a first data indicator with sections of the first one of theother storage areas, where the first data indicator for a particularsection of the first other one of the storage areas indicates whether awrite operation has been performed to the particular section after thefirst one of the other storage areas has been associated with the firstvirtual storage area, and associating a second data indicator withsections of the first one of the other storage areas, where the seconddata indicator for a particular section of the first other one of thestorage areas indicates whether a write operation has been performed tothe particular section after the second one of the other storage areashas been associated with the second virtual storage area. Accessingstored data may also include, in response to a write to a first sectionthat is on the first one of the other storage areas and the dataindicator indicating that no write operation has been performed to thefirst section after the first one of the other storage areas has beenassociated with the first virtual storage area, copying data of thefirst section to a second section that is on a second one of the otherstorage areas and causing a corresponding one of the pointers of thefirst table to point to the second section. Accessing stored data mayalso include, in response to a write to a first section that is on thefirst one of the other storage areas and the data indicator indicatingthat no write operation has been performed to the first section afterthe first one of the other storage areas has been associated with thefirst and second virtual storage areas, copying data of the firstsection to a second section that is on a second one of the other storageareas and causing a corresponding one of the pointers of the first tableand a corresponding one of the pointers of the second table to point tothe second section. Accessing stored data may also include, in responseto a write to the first virtual storage area corresponding to the secondsection, copying data from the second section to a third section.Accessing stored data may also include causing a corresponding one ofthe pointers of the first table to point to the third section. Accessingstored data may also include causing a corresponding one of the pointersof the second table to point to the third section. The first time may bethe same as the second time. The first time may be different from thesecond time. Each of the sections of data may be a track of data. Eachof the storage areas may be a storage device.

According further to the present invention, a computer program productincludes executable code that provides a virtual storage area having atable of pointers that point to sections of at least two other storageareas, where the virtual storage area contains no sections of data,executable code that determines which particular one of the otherstorage areas contain the data in response to a request for accessingdata of the virtual storage area, and executable code that accesses thedata on the particular one of the other storage areas using the table ofpointers. The computer program product may also include executable codethat associates a first one of the other storage areas with the virtualstorage area, where the virtual storage area represents a copy of dataof the first one of the other storage areas. The computer programproduct may also include executable code that causes all of the pointersof the table to initially point to sections of the first one of theother storage areas when the virtual storage area is initiallyassociated with the first one of the other storage areas. The computerprogram product may also include executable code that copies data of thefirst section to a second section that is on a second one of the otherstorage areas and causes a corresponding one of the pointers of thetable to point to the second section in response to a write to a firstsection on the first one of the other storage areas. The computerprogram product may also include executable code that associates a dataindicator with sections of the first one of the other storage areas,where the data indicator for a particular section indicates whether awrite operation has been performed to the particular section after thefirst one of the other storage areas has been associated with thevirtual storage area. The computer program product may also includeexecutable code that copies data of the first section to a secondsection that is on a second one of the other storage areas and causes acorresponding one of the pointers of the table to point to the secondsection in response to a write to a first section on the first one ofthe other storage areas and the data indicator indicating that no writeoperation has been performed to the first section after the first one ofthe other storage areas has been associated with the virtual storagearea. The computer program product may also include executable code thatsends status information to a device that caused the write operation tobe performed following copying the data from the first section to thesecond section. Each of the sections of data may be a track of data.Each of the storage areas may be a storage device.

According further to the present invention, a virtual storage deviceincludes at least one table for associating the virtual storage devicewith a standard storage device, storage for sections of data of thevirtual storage device, where a first portion of the storage forsections is sections of data of the standard storage device, and a firstplurality of pointers provided with the at least one table, where atleast some of the pointers point to sections of the standard storagedevice corresponding to the first portion. The virtual storage devicemay also include a second portion of the storage for sections thatcontain data that is different from data on corresponding sections ofthe standard storage device that map to the second portion, and a secondplurality of pointers provided with the at least one table, where thesecond plurality of pointers point to sections of a device differentfrom the standard storage device. Each section of the second portion maycontain an earlier version of data on a corresponding section of thestandard storage device. Each of the sections of data may be a track ofdata.

According to the present invention, a host computer to establishes acoupling between a logical storage area of a storage device and avirtual storage area of the storage device, by sending a first commandto the storage device to register the logical storage area, followingthe first command, the host computer sending a second command to thestorage device to relate the logical storage area to the virtual storagearea, and following the second command, the host computer sending athird command to the storage device to activate the coupling between thelogical storage area and the virtual storage area, where followingactivating the coupling, pointers of the virtual storage area point tosections of the logical storage area and where a write to the logicalstorage area causes data to be copied from the logical storage area toan other area of the storage device and causes a corresponding pointerof the virtual storage area to point to the other area. The firstcommand may cause creation of the virtual storage area. The secondcommand may cause creation of the virtual storage area. The secondcommand may include additional parameters, which may be selected fromthe group consisting of: a new name for the virtual storage area and anindicator that determines whether the virtual storage area is availableto a host. The storage areas may be devices.

According further to the present invention, a host computer establishesa coupling between at least one of a plurality of logical storage areasof a storage device and at least one of a corresponding one of aplurality of virtual storage areas of the storage device by sendingcommands to the storage device to relate the at least one of the logicalstorage areas to a corresponding one of the virtual storage areas andby, following sending commands, causing an activation of couplingsbetween at least one of the plurality of the logical storage areas andcorresponding ones of the plurality of virtual storage areas, wherefollowing activation, pointers of at least one of the virtual storageareas corresponding to the couplings point to sections of acorresponding one of the logical storage areas and where a write to thecorresponding one of the logical storage areas causes data to be copiedfrom the corresponding one of logical storage areas to an other area ofthe storage device and causes a corresponding pointer of the at leastone of the virtual storage areas to point to the other area. The hostsending commands may cause creation of the at least one of the virtualstorage areas. Causing activation may include the host sendingadditional commands. The additional commands may include parameters,which may be selected from the group consisting of: a new name for atleast one of the virtual storage areas and an indicator that determineswhether at least one of the virtual storage areas is available to ahost. A host computer establishing a coupling between at least one of aplurality of logical storage areas of a storage device and at least oneof a corresponding one of a plurality of virtual storage areas of thestorage device may also include the host maintaining a list of thelogical storage areas and the virtual storage areas. Activation ofcouplings may include passing the list from the host to the storagedevice. The list may be a linked list. Passing the list may includepassing a pointer to a data structure that contains one of: the list anda pointer to the list. The logical storage areas may all be part of asingle consistency group. Causing an activation of couplings may includethe host providing one activate command to the storage device for eachlogical storage area/virtual storage area pair. Causing an activation ofcouplings may include the host providing a single activate command tothe storage device that activates all of the logical storagearea/virtual storage area pairs. The storage areas may be devices.

According further to the present invention, computer software thatestablishes a coupling between a logical storage area of a storagedevice and a virtual storage area of the storage device includesexecutable code that sends a first command to the storage device toregister the logical storage area, executable code that sends a secondcommand to the storage device to relate the logical storage area to thevirtual storage area following sending the first command, and executablecode that sends a third command to the storage device to activate thecoupling between the logical storage area and the virtual storage areafollowing the second command, where following activating the coupling,pointers of the virtual storage area point to sections of the logicalstorage area and where a write to the logical storage area causes datato be copied from the logical storage area to an other area of thestorage device and causes a corresponding pointer of the virtual storagearea to point to the other area. The second command may includeadditional parameters, which may be selected from the group consistingof: a new name for the virtual storage area and an indicator thatdetermines whether the virtual storage area is available to a host.

According further to the present invention, computer software thatestablishes a coupling between at least one of a plurality of logicalstorage areas of a storage device and at least one of a correspondingone of a plurality of virtual storage areas of the storage device,includes executable code that sends commands to the storage device torelate the at least one of the logical storage areas to a correspondingone of the virtual storage areas and executable code that causes anactivation of couplings between at least one of the plurality of thelogical storage areas and corresponding ones of the plurality of virtualstorage areas after the commands are sent, where following activation,pointers of at least one of the virtual storage areas corresponding tothe couplings point to sections of a corresponding one of the logicalstorage areas and where a write to the corresponding one of the logicalstorage areas causes data to be copied from the corresponding one oflogical storage areas to an other area of the storage device and causesa corresponding pointer of the at least one of the virtual storage areasto point to the other area. Executable code that causes activation maysend additional commands. The additional commands may include parametersthat may be selected from the group consisting of: a new name for atleast one of the virtual storage areas and an indicator that determineswhether at least one of the virtual storage areas is available to ahost. The computer software may also include executable code thatmaintains a list of the logical storage areas and the virtual storageareas.

According further to the present invention, handling data includesproviding a first storage area of a first type that contains sections ofdata, providing a second storage area of the first type that containssections of data, providing a third storage area of a second type wherethe second type has, for each section thereof, a pointer to one of: acorresponding section of data of the first storage area and acorresponding section of data of the second storage area, causing thethird storage area to be not available for accessing, and after causingthe third storage area to not be available for accessing, providing datafrom the second storage area corresponding to pointers of the thirdstorage area that point to sections of the second storage area. Handlingdata may also include, after providing the storage areas and prior tocausing the third storage area to not be available, handling a write toa particular section of the first storage area pointed to by acorresponding pointer of the third storage area by copying data from theparticular section of the first storage area to a corresponding sectionof the second storage area and adjusting the corresponding pointer ofthe third storage area to point to the corresponding section of thesecond storage area. Handling data may also include, after causing thethird storage area to not be available, handling a write to a particularsection of the first storage area by writing the data thereto. Causingthe third storage area to not be available may include providing a valuein a header for the first storage area, where the value indicates thatno operation is to be performed in connection with a set protection bitencountered when data is written to a corresponding section of the firststorage area. Handling data may also include inhibiting access to thefirst storage area prior to providing the value to the header andallowing access to the first storage area after providing the value tothe header. The storage areas may be devices.

According further to the present invention, retrieving requested datafrom a virtual storage area includes determining if the virtual storagearea is deactivated, if the virtual storage area is deactivated,determining if the requested data corresponds to data handled by thevirtual storage area prior to being deactivated, and if the requesteddata corresponds to data handled by the virtual storage area prior tobeing deactivated, providing the requested data. Determining if therequested data corresponds to data handled by the virtual storage areaprior to being deactivated may include examining protection bits of acorresponding standard logical storage area. Providing the requesteddata may include reading the virtual storage area. The storage areas maybe devices.

According further to the present invention, restoring data to a previousversion, includes obtaining a current version of the data, obtainingpreviously archived sections of the data, and iteratively applying thepreviously archived sections of data to the current version and toresulting intermediate versions until the data corresponds to theprevious version of the data. Previously archived sections maycorrespond to versions of the sections that existed prior to archiving.Archiving may include providing a virtual storage area containingpointers to sections of data of a logical storage area and, in responseto a write to a section of the logical storage area, copying data fromthe logical storage area to an other area and causing the virtualstorage area to point to the other area, where the other area containsthe archived data.

According further to the present invention, computer software handlesdata used in connection with a first storage area of a first type thatcontains sections of data, a second storage area of the first type thatcontains sections of data and a third storage area of a second typewherein the second type has, for each section thereof, a pointer to oneof: a corresponding section of data of the first storage area and acorresponding section of data of the second storage area and thesoftware includes executable code that causes the third storage area tobe not available for accessing and executable code that, after the thirdstorage area is not available for accessing, provides data from thesecond storage area corresponding to pointers of the third storage areathat point to sections of the second storage area. Executable code thatcauses the third storage area to not be available may include executablecode that provides a value in a header for the first storage area, wherethe value indicates that no operation is to be performed in connectionwith a set protection bit encountered when data is written to acorresponding section of the first storage area.

According further to the present invention, computer software thatretrieves requested data from a virtual storage area includes executablecode that determines if the virtual storage area is deactivated,executable code that, if the virtual storage area is deactivated,determines if the requested data corresponds to data handled by thevirtual storage area prior to being deactivated, and executable codethat, if the requested data corresponds to data handled by the virtualstorage area prior to being deactivated, provides the requested data.The software may also include executable code that examines protectionbits of a corresponding standard logical storage area. The software mayalso include executable code that reads the virtual storage area.

According further to the present invention, computer software thatrestores data to a previous version includes executable code thatobtains a current version of the data, executable code that obtainspreviously archived sections of the data, and executable code thatiteratively applies the previously archived sections of data to thecurrent version and to resulting intermediate versions until the datacorresponds to the previous version of the data. Previously archivedsections may correspond to versions of the sections that existed priorto archiving.

According further to the present invention, accessing data includes ahost establishing a relationship between a first storage area of a firsttype containing data and a second storage area of a second typecontaining pointers to data provided in storage areas of the first type,where the storage areas are provided in a storage device coupled to thehost, in response to the host writing data to a particular section ofthe first storage area after establishing the relationship, theparticular section being copied from the first storage area to a thirdstorage area of the first type prior to the write operation beingexecuted, and a corresponding pointer of the second storage area beingadjusted to point to the third storage area. Accessing data may alsoinclude restoring data from the second storage area to the first storagearea. Accessing data may also include restoring data from the secondstorage area to a fourth storage area of the first type. The fourthstorage area may be a split mirror of the first storage area. The firstand second storage areas may be storage devices. Establishing arelationship may include the host providing an optional new name for thesecond storage device. Establishing a relationship may include the hostproviding an optional online/offline parameter for the second storagedevice. Accessing data may also include deactivating the second storagearea by making the second storage area not available to the host.Accessing data may also include making sections copied from the firststorage area to the third storage area available to the host. Thestorage areas may be devices.

According further to the present invention, providing a virtual storagearea containing no sections of data, includes providing first and secondstorage areas, each containing sections of data, and providing aplurality of pointers, where each pointer points to one of: a section ofthe first storage area and a section of the second storage area. Thevirtual storage area may represent a point in time copy of the firststorage area. Prior to writing data to a particular section of the firststorage area pointed to by a particular pointer of the virtual storagearea, the particular section may be copied to the second storage areaand the particular pointer may be made to point thereto. The pointers,virtual storage area, first storage area, and second storage area mayall be provided on a storage device. Data corresponding to the virtualstorage area may be accessed by a plurality of host processors coupledto the storage device. The storage areas may be storage devices.

According further to the present invention, accessing data stored inconnection with a virtual storage area containing no sections of dataincludes obtaining a pointer of the virtual storage area correspondingto the data, in response to the pointer pointing to a first storagearea, accessing the first storage area, and in response to the pointerpointing to a second storage area, accessing the second storage area.The pointers, virtual storage area, first storage area, and secondstorage area may all be provided on a storage device. Data correspondingto the virtual storage area may be accessed by a plurality of hostprocessors coupled to the storage device. The storage areas may bedevices.

According further to the present invention, computer software thatprovides a virtual storage area containing no sections of data, includesexecutable code that provides first and second storage areas, eachcontaining sections of data and executable code that provides aplurality of pointers, wherein each pointer points to one of: a sectionof the first storage area and a section of the second storage area. Thevirtual storage area may represent a point in time copy of the firststorage area. The computer software may also include executable codethat, prior to writing data to a particular section of the first storagearea pointed to by a particular pointer of the virtual storage area,executable code that copies the particular section to the second storagearea and adjusts the particular pointer to point thereto.

According further to the present invention, computer software thataccesses data stored in connection with a virtual storage areacontaining no sections of data, includes executable code that obtains apointer of the virtual storage area corresponding to the data,executable code that, in response to the pointer pointing to a firststorage area, accesses the first storage area, and executable code that,in response to the pointer pointing to a second storage area, accessesthe second storage area.

According further to the present invention, establishing a plurality ofstorage areas includes associating each of a first plurality of storageareas of a first type that contain sections of data with correspondingones of second plurality of storage areas of a second type havingpointers to alternative sections of data storage areas of the firsttype, wherein initially none of the second plurality of storage areas isavailable for accessing data corresponding thereto, and, afterassociating all of the first and second plurality of storage areas,activating the second plurality of storage areas to make the secondplurality of storage areas available for accessing data. Establishing aplurality of storage areas may also include, prior to activating thesecond plurality of storage areas, setting pointers of the secondplurality of storage areas to point to sections of corresponding ones ofthe first plurality of storage areas. Establishing a plurality ofstorage areas may also include, after activating the second plurality ofstorage areas, responding to a write to one of the first plurality ofstorage areas by copying a portion of data thereof to the alternativesections of data storage areas and adjusting a pointer of acorresponding one of the second plurality of storage areas to point tothe alternative sections of data storage areas. Establishing a pluralityof storage areas may also include enabling exclusive access to the firstplurality of storage areas prior to activating the second plurality ofstorage areas, and disabling exclusive access to the first plurality ofstorage areas after activating the second plurality of storage areas.Establishing a plurality of storage areas may also include enablingexclusive access to one of the first plurality of storage areas prior toactivating a corresponding one of the second plurality of storage areas,and disabling exclusive access to the one of the first plurality ofstorage areas after activating the corresponding one of the secondplurality of storage areas. Activating the second plurality of storageareas may include providing a value in device headers for each of thesecond plurality of storage areas. Establishing a plurality of storageareas may also include establishing a consistency group for the logicalstorage areas and the virtual storage areas. Associating each of thefirst plurality of storage areas with corresponding ones of the secondplurality of storage areas may include making a list of associatedstorage areas that is used in connection with activating the storageareas. The list may be a linked list. Activating the second plurality ofstorage areas may includes setting a particular protection bit for eachsection of each of the first plurality of storage areas, and setting avalue in each of the headers for each of the first plurality of storageareas, wherein the value indicates that special processing is to beperformed in response to a write to a section of the first plurality ofstorage areas having a protection bit set. The storage areas may bedevices.

According to the present invention, computer software that establishes aplurality of storage areas includes executable code that associates eachof a first plurality of storage areas of a first type that containsections of data with corresponding ones of second plurality of storageareas of a second type having pointers to alternative sections of datastorage areas of the first type, where initially none of the secondplurality of storage areas is available for accessing data correspondingthereto and executable code that, after associating all of the first andsecond plurality of storage areas, activates the second plurality ofstorage areas to make the second plurality of storage areas availablefor accessing data. The computer software may also include executablecode that, prior to activating the second plurality of storage areas,sets pointers of the second plurality of storage areas to point tosections of corresponding ones of the first plurality of storage areas.The computer software may also include executable code that, afteractivating the second plurality of storage areas, responds to a write toone of the first plurality of storage areas by copying a portion of datathereof to the alternative sections of data storage areas and adjustinga pointer of a corresponding one of the second plurality of storageareas to point to the alternative sections of data storage areas. Thecomputer software may also include executable code that enablesexclusive access to the first plurality of storage areas prior toactivating the second plurality of storage areas and executable codethat disables exclusive access to the first plurality of storage areasafter activating the second plurality of storage areas. The computersoftware may also include executable code that enables exclusive accessto one of the first plurality of storage areas prior to activating acorresponding one of the second plurality of storage areas andexecutable code that disables exclusive access to the one of the firstplurality of storage areas after activating the corresponding one of thesecond plurality of storage areas. Executable code that activates thesecond plurality of storage areas may include executable code thatprovides a value in device headers for each of the second plurality ofstorage areas. The computer software may also include executable codethat establishes a consistency group for the logical storage areas andthe virtual storage areas. Executable code that associates each of thefirst plurality of storage areas with corresponding ones of the secondplurality of storage areas may also make a list of associated storageareas that is used in connection with activating the storage areas. Thelist may be a linked list. Executable code that activates the secondplurality of storage areas may also set a particular protection bit foreach section of each of the first plurality of storage areas and mayalso set a value in each of the headers for each of the first pluralityof storage areas, where the value indicates that special processing isto be performed in response to a write to a section of the firstplurality of storage areas having a protection bit set.

According further to the present invention, restoring data includesproviding data in a first storage area of a first type that containssections of data, providing data in a second storage area of a secondtype wherein the second type has, for each section of data thereof, atleast one of: a pointer to a corresponding section of data of the firststorage area and a pointer to corresponding section of data of a thirdstorage area of the first type, and, for each particular section of dataof the second storage area having a pointer to the third storage area,replacing a corresponding section of the first storage area with apointer to the third storage area. Restoring data may also include,after replacing all of the particular sections of the first storagearea, deallocating the second storage area. Restoring data may alsoinclude, after replacing all of the particular sections of the firststorage area, causing data to be copied from the third storage area tothe first storage area. The storage areas may be devices.

According further to the present invention, restoring data includesproviding data in a first storage area of a first type that containssections of data, providing data in a second storage area of a secondtype wherein the second type has, for each section of data thereof, atleast one of: a pointer to a corresponding section of data of the firststorage area and a pointer to corresponding section of data of a thirdstorage area of the first type, for each particular section of data ofthe second storage area having a pointer to the third storage area,replacing a corresponding section of a fourth storage area of the firsttype with the pointer to the third storage area, and, for eachparticular section of data of the second storage area having a pointerto the first storage area, replacing a corresponding section of thefourth storage area with a pointer to the first storage area. Restoringdata may also include, after replacing all of the particular sections ofthe fourth storage area, deallocating the second storage area. Restoringdata may also include, after replacing all of the particular sections ofthe fourth storage area, causing data to be copied from the first andthird storage areas to the fourth storage area. The storage areas may bedevices.

According further to the present invention, restoring data includesproviding data in a first storage area of a first type that containssections of data, providing data in a second storage area of a secondtype where the second type has, for each section of data thereof, atleast one of: a pointer to a corresponding section of data of the firststorage area and a pointer to corresponding section of data of a thirdstorage area of the first type, and, for each particular section of dataof the second storage area having a pointer to the third storage area,replacing a corresponding section of a fourth storage area of the firsttype with a pointer to the third storage area, where the fourth storagearea is at least a partial mirror copy of the first storage area.Restoring data may also include, after replacing all of the particularsections of the fourth storage area, deallocating the second storagearea. Restoring data may also include, after replacing all of theparticular sections of the fourth storage area, causing data to becopied from the third storage area to the fourth storage area. Thestorage areas may be devices.

According further to the present invention, restoring data includesproviding data in a first storage area of a first type that containssections of data, providing data in a second storage area of a secondtype wherein the second type has, for each section of data thereof, atleast one of: a pointer to a corresponding section of data of the firststorage area and a pointer to corresponding section of data of a thirdstorage area of the first type, and copying data from the second storagearea to a third storage area of the second type. Restoring data may alsoinclude, after copying data, deallocating the second storage area.

According further to the present invention, computer software restoresdata, in connection with a system that provides data in a first storagearea of a first type that contains sections of data and provides data ina second storage area of a second type wherein the second type has, foreach section of data thereof, at least one of: a pointer to acorresponding section of data of the first storage area and a pointer tocorresponding section of data of a third storage area of the first type,and the software includes executable code that, for each particularsection of data of the second storage area having a pointer to the thirdstorage area, replaces a corresponding section of the first storage areawith a pointer to the third storage area. The software may also includeexecutable code that, after replacing all of the particular sections ofthe first storage area, causes data to be copied from the third storagearea to the first storage area.

According further to the present invention, computer software restoresdata in connection with a system that provides data in a first storagearea of a first type that contains sections of data and provides data ina second storage area of a second type wherein the second type has, foreach section of data thereof, at least one of: a pointer to acorresponding section of data of the first storage area and a pointer tocorresponding section of data of a third storage area of the first type,and the software includes executable code that, for each particularsection of data of the second storage area having a pointer to the thirdstorage area, replaces a corresponding section of a fourth storage areaof the first type with the pointer to the third storage area, andexecutable code that, for each particular section of data of the secondstorage area having a pointer to the first storage area, replaces acorresponding section of the fourth storage area with a pointer to thefirst storage area. The computer software also includes executable codethat, after replacing all of the particular sections of the fourthstorage area, causes data to be copied from the first and third storageareas to the fourth storage area.

According further to the present invention computer software restoresdata in connection with a system that provides data in a first storagearea of a first type that contains sections of data and provides data ina second storage area of a second type wherein the second type has, foreach section of data thereof, at least one of: a pointer to acorresponding section of data of the first storage area and a pointer tocorresponding section of data of a third storage area of the first type,and the software includes executable code that, for each particularsection of data of the second storage area having a pointer to the thirdstorage area, replacing a corresponding section of a fourth storage areaof the first type with a pointer to the third storage area, where thefourth storage area is at least a partial mirror copy of the firststorage area. The computer software also includes executable code that,after replacing all of the particular sections of the fourth storagearea, causes data to be copied from the third storage area to the fourthstorage area.

According further to the present invention, computer software restoresdata in connection with a system that provides data in a first storagearea of a first type that contains sections of data and provides data ina second storage area of a second type wherein the second type has, foreach section of data thereof, at least one of: a pointer to acorresponding section of data of the first storage area and a pointer tocorresponding section of data of a third storage area of the first typeand the software includes executable code that copies data from thesecond storage area to a third storage area of the second type.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of a storage device used in connection with thesystem described herein.

FIG. 2 is a diagram of a storage that shows various logical volumes thatare used in connection with the system described herein.

FIG. 3 is a diagram showing use of a virtual device according to thesystem described herein.

FIG. 4 is a diagram showing use of a plurality of virtual devicesaccording to the system described herein.

FIG. 5 is a diagram showing device tables used in connection with thesystem described herein.

FIG. 6 is a flow chart illustrating reading a table used in connectionwith a virtual device according to the system described herein.

FIG. 7 is a flow chart illustrating writing to a table used inconnection with a virtual device according to the system describedherein.

FIG. 8 is a flow chart illustrating modification of a virtual devicetable and establishing a virtual device according to the systemdescribed herein.

FIG. 9 is a flow chart illustrating modification of data structures usedto handle tracks of a log device according to the system describedherein.

FIG. 10 is a flow chart illustrating steps performed in connection withreading a virtual device according to the system described herein.

FIG. 11 is a flow chart illustrating steps performed by a disk adapterin connection with writing to a standard logical device to which avirtual device has been established according to the system describedherein.

FIG. 12 is a flow chart illustrating steps performed by a host adapterin connection with writing to a standard logical device to which avirtual device has been established according to the system describedherein.

FIG. 13 is a flow chart illustrating steps performed in connection withwriting to a virtual device according to the system described herein.

FIG. 14 is a flow chart illustrating steps performed in connection withremoving a virtual device.

FIGS. 15A, 15B, and 15C are flow charts illustrating modifyingprotection bits of tracks of a storage device according to an embodimentof the system described herein.

FIGS. 16A and 16B are portions of flow charts illustrating alternativeprocessing for the flow charts of FIGS. 11 and 12, respectively, whenprotection bits are set according to the flow charts of FIGS. 15A, 15B,and 15C.

FIG. 17 is a flow chart illustrating registering a standard logicaldevice/virtual device pair according to the system described herein.

FIG. 18 is a flow chart illustrating relating a standard logical deviceto a virtual device according to the system described herein.

FIG. 19A is a flow chart illustrating steps performed in connection withactivation of one or more standard logical device/virtual device pairsaccording to the system described herein.

FIG. 19B is a flow chart illustrating steps performed in connection withactivation of one or more standard logical device/virtual device pairsaccording to another embodiment of the system described herein.

FIGS. 20A, 20B, 20C, 20D, and 20E illustrate different ways to restore avirtual device to a standard logical device or another virtual deviceaccording to the system described herein.

FIG. 21 is a flow chart that illustrates steps performed to restore avirtual logical device to a standard logical device according to a firstembodiment of the system described herein.

FIG. 22 is a flow chart that illustrates steps performed to restore avirtual logical device to a standard logical device according to asecond embodiment of the system described herein.

FIG. 23 is a flow chart that illustrates steps performed to restore avirtual logical device to a split mirror standard logical deviceaccording to a third embodiment of the system described herein.

FIG. 24 is a flow chart illustrating interconnects between hosts and astorage device according to the system described herein.

FIG. 25 is a flow chart illustrating a host application call toestablish a standard logical device/virtual device pair according to thesystem described herein.

FIG. 26 is a flow chart illustrating a host application call to restorea virtual device to a standard logical device according to the systemdescribed herein.

FIG. 27 is a flow chart illustrating deactivating a virtual deviceaccording to the system described herein.

FIG. 28 is a flow chart illustrating obtaining data for deactivated fromtracks of a deactivated virtual device according to the system describedherein.

FIG. 29 is a flow chart illustrating steps performed in connection withproviding a rolling back up according to the system described herein.

FIG. 30 is a flow chart illustrating restoring data from a rolling backup according to the system described herein.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Referring to FIG. 1, a storage device 30 includes a plurality of hostadapters (HA) 32-34, a plurality of disk adapters (DA) 36-38 and aplurality of disk drives 42-44. Each of the disk drives 42-44 is coupledto a corresponding one of the DA's 36-38. The storage device 30 alsoincludes a global memory 46 that may be accessed by the HA's 32-34 andthe DA's 36-38. The storage device 30 also includes a RDF adapter (RA)48 that may also access the global memory 46. The RA 48 may communicatewith one or more additional remote storage devices (not shown) and/orone or more other remote devices (not shown) via a data link 52. TheHA's 32-34, the DA's 36-38, the global memory 46 and the RA 48 arecoupled to a bus 54 that is provided to facilitate communicationtherebetween.

Each of the HA's 32-34 may be coupled to one or more host computers (notshown) that access the storage device 30. The host computers (hosts)read data stored on the disk drives 42-44 and write data to the diskdrives 42-44. The global memory 46 contains a cache memory that holdstracks of data from the disk drives 42-44 as well as storage for tablesthat may be accessed by the HA's 32-34, the DA's 36-38 and the RA 48.Note that, for the discussion herein, blocks of data are described asbeing a track or tracks of data. However, it will be appreciated by oneof ordinary skill in the art, that the system described herein may workwith any appropriate incremental amount, or section, of data, includingpossibly variable incremental amounts of data and/or fixed incrementalamounts of data.

Referring to FIG. 2, the storage device 30 is shown as including aplurality of standard logical devices 61-68. Each of the standardlogical devices 61-68 may correspond to a volume that is accessible toone or more hosts coupled to the storage device 30. Each of the standardlogical devices 61-68 may or may not correspond to one of the diskdrives 42-44. Thus, for example, the standard logical device 61 maycorrespond to the disk drive 42, may correspond to a portion of the diskdrive 42, or may correspond to a portion of the disk drive 42 and aportion of the disk drive 43. Each of the standard logical devices 61-68appears to the host as a contiguous block of disk storage, even thougheach of the standard logical devices 61-68 may or may not correspond toactual contiguous physical storage of the disk drives 42-44.

The storage device 30 may also includes a plurality of virtual devices71-74. The virtual devices 71-74 appear to a host coupled to the storagedevice 30 as volumes containing a contiguous block of data storage. Eachof the virtual devices 71-74 may represent a point in time copy of anentire one of the standard logical devices 61-68, a portion of one ofthe standard logical devices 61-68, or a combination of portions orentire ones of the standard logical devices 61-68. However, as describedin more detail elsewhere herein, the virtual devices 71-74 do notcontain the track data from the standard logical devices 61-68. Instead,each of the virtual devices 71-74 is coupled to a log device 76 or a logdevice 78 that stores some or all the track data, as described in moredetail elsewhere herein. The virtual devices 71-74 contain tables thatpoint to tracks of data on either on the standard logical devices 61-68or the log devices 76, 78.

The virtual device 71 may represent a point in time copy of the standardlogical device 61. As described in more detail elsewhere herein, thevirtual device 71 is coupled to the log device 76 that contains trackdata to facilitate the virtual device 71 appearing to a host to be apoint in time copy of the standard logical device 61. It is possible formore than one virtual device to use a single log device. Thus, thevirtual devices 72-74 are shown being coupled to the log device 78.Similarly, it is possible for more than one virtual device to representpoint in time copies of a single standard logical device. Thus, thevirtual devices 72,73 are shown as being point in time copies of thestandard logical device 64. The virtual devices 72,73 may represent thesame point in time copy of the standard logical device 64 or,alternatively, may represent point in time copies of the standardlogical device 64 taken at different times. Note that only some of thestandard logical devices 61-68 are shown as being associated with acorresponding one of the virtual devices 71-74 while others of thestandard logical devices 61-68 are not.

In some embodiments, it may be possible to implement the systemdescribed herein using storage areas, instead of storage devices. Thus,for example, the virtual devices 71-74 may be virtual storage areas, thestandard logical devices 61-68 may be standard logical areas, and thelog devices 76,78 may be log areas. In some instances, such animplementation may allow for hybrid logical/virtual devices where asingle logical device has portions that behave as a standard logicaldevice, portions that behave as a virtual device, and/or portions thatbehave as log device. Accordingly, it should be understood that, inappropriate instances, references to devices in the discussion hereinmay also apply to storage areas that may or may not correspond directlywith a storage device.

Referring to FIG. 3, a diagram shows a standard logical device 82, avirtual device 84, and a log device 86. As discussed above, the virtualdevice 84 may represent a point in time copy of all or a portion of thestandard logical device 82. A host coupled to a storage device thataccesses the virtual device 84 may access the virtual device 84 in thesame way that the host would access the standard logical device 82.However, the virtual device 84 does not contain any track data from thestandard logical device 82. Instead, the virtual device 84 includes aplurality of table entries that point to tracks on either the standardlogical device 82 or the log device 86.

When the virtual device is established 84 (e.g., when a point in timecopy is made of the standard logical device 82), the virtual device 84is created and provided with appropriate table entries that, at the timeof establishment, point to tracks of the standard logical device 82. Ahost accessing the virtual device 84 to read a track would read theappropriate track from the standard logical device 82 based on the tableentry of the virtual device 84 pointing to the track of the standardlogical device 82.

After the virtual device 84 has been established, it is possible for ahost to write data to the standard logical device 82. In that case, theprevious data that was stored on the standard logical device 82 iscopied to the log device 86 and the table entries of the virtual device84 that previously pointed to tracks of the standard logical device 82would be modified to point to the new tracks of the log device 86 towhich the data had been copied. Thus, a host accessing the virtualdevice 84 would read either tracks from the standard logical device 82that have not changed since the virtual device 84 was established or,alternatively, would read corresponding tracks from the log device 86that contain data copied from the standard logical device 82 after thevirtual device 84 was established. Adjusting data and pointers inconnection with reads and writes to and from the standard logical device82 and virtual device 84 is discussed in more detail elsewhere herein.

In an embodiment described herein, hosts would not have direct access tothe log device 86. That is, the log device 86 would be used exclusivelyin connection with the virtual device 84 (and possibly other virtualdevices as described in more detail elsewhere herein). In addition, foran embodiment described herein, the standard logical device 82, thevirtual device 84, and the log device 86 may be provided on the singlestorage device 30. However, it is possible to provide the differentlogical devices and the log device on separate storage devicesinterconnected using, for example, the RDF protocol or other remotecommunication protocols. In addition, it may be possible to haveportions of one or more of the standard logical device 82, the virtualdevice 84, and/or the log device 86 provided on separate storage devicesthat are appropriately interconnected.

Referring to FIG. 4, another example of the use of virtual devices showsa standard logical device 92, a plurality of virtual devices 94-97 and alog device 98. In the example of FIG. 4, the virtual device 94represents a point in time copy of the standard logical device 92 takenat ten a.m. Similarly, the virtual device 95 represents a copy of thestandard logical device 92 taken at twelve noon, the virtual device 96represents a copy of the standard logical device 92 taken at two p.m.,and the virtual device 97 represents a copy of the standard logicaldevice 92 taken at four p.m. Note that all of the virtual devices 94-97may share the log device 98. In addition, it is possible for tableentries of more than one of the virtual devices 94-97, or, a subset ofthe table entries of the virtual devices 94-97, to point to the sametracks of the log device 98. For example, the virtual device 95 and thevirtual device 96 are shown as having table entries that point to thesame tracks of the log device 98.

In an embodiment discussed herein, the log device 98 and other logdevices discussed herein are provided by a pool of log devices that ismanaged by the storage device 30. In that case, as a virtual devicerequires additional tracks of a log device, the virtual device wouldcause more log device storage to be created (in the form of more tracksfor an existing log device or a new log device) using the log devicepool mechanism. Pooling storage device resources in this manner is knownin the art. Other techniques that do not use pooling may be used toprovide log device storage.

Referring to FIG. 5, a diagram 100 illustrates tables that are used tokeep track of device information. A first table 102 corresponds to allof the devices used by a storage device or by an element of a storagedevice, such as an HA and/or a DA. The table 102 includes a plurality oflogical device entries 106-108 that correspond to all the logicaldevices used by the storage device (or portion of the storage device).The entries in the table 102 include descriptions for standard logicaldevices, virtual devices, log devices, and other types of logicaldevices.

Each of the entries 106-108 of the table 102 correspond to another tablethat contains information for each of the logical devices. For example,the entry 107 may correspond to a table 112. The table 112 includes aheader that contains overhead information. The table 112 also includesentries 116-118 for each of the cylinders of the logical device. In anembodiment disclosed herein, a logical device may contain any number ofcylinders depending upon how the logical device is initialized. However,in other embodiments, a logical device may contain a fixed number ofcylinders.

The table 112 is shown as including a section for extra track bytes 119.The extra track bytes 119 are used in connection with the log devices ina manner that is discussed elsewhere herein. In an embodiment disclosedherein, there are eight extra track bytes for each track of a logdevice. For devices that are not log devices, the extra track bytes 119may not be used.

Each of the cylinder entries 116-118 corresponds to a track table. Forexample, the entry 117 may correspond to a track table 122 that includesa header 124 having overhead information. The track table 122 alsoincludes entries 126-128 for each of the tracks. In an embodimentdisclosed herein, there are fifteen tracks for every cylinder. However,for other embodiments, it may be possible to have different numbers oftracks for each of the cylinders or even a variable number of tracks foreach cylinder. For standard logical devices and log devices, theinformation in each of the entries 126-128 includes a pointer (eitherdirect or indirect) to the physical address on one of the disk drives42-44 of the storage device 30 (or a remote storage device if the systemis so configured). Thus, the track table 122 may be used to map logicaladdresses of the logical device corresponding to the tables 102, 112,122 to physical addresses on the disk drives 42-44 of the storage device30. For virtual devices, each of the entries 126-128 of the table 122points to a track of a corresponding standard logical device orcorresponding log device. For other embodiments, however, it may bepossible to use a different mechanism where the tables 102, 122, 122 areused only for standard logical devices that contain tracks of data whileanother type of table, such as a simple array of tracks, is used byvirtual devices to map tracks of the virtual devices to tracks ofcorresponding standard logical devices or log devices.

Each track of a log device is either free, meaning that it is not beingused by a virtual device, or is assigned, meaning that the track ispointed to by a table entry in one or more of the virtual devices. In anembodiment disclosed herein, the tracks of a log device are managed byfirst creating a doubly linked list of all of the free tracks of the logdevice. The pointers for the doubly linked list are provided by theextra track bytes 119 of the table 112 so that the extra track bytes 119for a log device contains eight bytes for every track of the log device.For every track of the log device that is free, the extra eight bytesinclude a forward pointer pointing to the next free track of the logdevice and a backward pointer pointing to the previous free track of thelog device. Using a doubly linked list in this manner facilitatesaccessing free tracks of the log device.

In addition, if a track of a log device is assigned (i.e., is used byone or more virtual devices), the corresponding extra track bytes 119for the track may be used to point back to the corresponding track ofthe standard logical device. Thus, when a write is performed to thestandard logical device after the virtual device has been established,the data from the standard logical device is copied to a new track ofthe log device and the extra track bytes corresponding to the new trackof the log device are made to point back to the track of the standardlogical device from which the data came. Having each track of the logdevice point back to the corresponding track of the standard logicaldevice is useful in, for example, data recovery situations.

In addition, for an embodiment disclosed herein, the pointers for theextra eight bytes per track for an assigned track are stored with thedata also. That is, when a particular track of a log device is assigned,the pointer back to the corresponding track of a standard logical deviceis stored with the extra track bytes 119 and, in addition, the pointeris stored with the track data itself on the track of the log device. ForCKD formatted tracks, the extra eight bytes may be stored in block zero.For FBA formatted tracks, the extra eight bytes may be stored in anadditional block appended on the end of the track. In an embodimentdisclosed herein, a block is five hundred and twelve bytes and an FBAtrack contains forty blocks, which is increased to forty-one when anadditional block is appended. Different track formats are disclosed, forexample, in U.S. Pat. No, 5,206,939 to Yanai, et al., which isincorporated herein by reference.

The tables 102, 112, 122 of FIG. 5 may be stored in the global memory 46of the storage device 30. In addition, the tables corresponding todevices accessed by a particular host may be stored in local memory ofthe corresponding one of the HA's 32-36. In addition, the RA 48 and/orthe DA's 36-38 may also use and locally store portions of the tables102, 112, 122.

Referring to FIG. 6, a flow chart 140 illustrates steps performed when ahost reads data from a device table corresponding to a track that isaccessible through a virtual device. That is, the flow chart 140illustrates obtaining information about a track that is pointed to by atable entry for a virtual device.

Processing begins at a test step 142 where it is determined if the trackof interest (i.e., the track corresponding to the table entry beingread) is on the standard logical device or the log device. This isdetermined by accessing the device table entry for the virtual deviceand determining whether the table entry for the track of interest pointsto either the standard logical device or the log device. If it isdetermined at the test step 142 that the pointer in the table for thevirtual device points to the standard logical device, then controlpasses from the step 142 to a step 148 where the table entry of interestis read. Following the step 148, processing is complete.

If it is determined that the test step 142 that the pointer in thedevice table for the virtual device for the track of interest points tothe log device, then control transfers from the step 142 to a step 158where the log table entry of interest is read. Following the step 158,processing is complete.

Note that, in some instances, access to data may be controlled by a flagor lock that prohibits multiple processes having access to the datasimultaneously. This is especially useful in instances where a devicetable is being read or modified. The system disclosed hereincontemplates any one of a variety of mechanisms for controlling accessto data by multiple processes, including conventional combinations ofsoftware and/or hardware locks, also known as “flags” or “semaphores”.In some instances, a process accessing data may need to wait untilanother process releases the data. In one embodiment, a hardware lockcontrols access to a software lock (flag) so that a process firstobtains control of the hardware lock, tests the software lock, and then,if the software lock is clear, the process sets the software lock andthen releases the hardware lock. If the process gets the hardware lockand determines that the software lock is not clear, then the processreleases the hardware lock so that another process that has set thesoftware lock can clear the software lock at a later time. Further notethat, in some instances, it is useful to first read a table entrycorresponding to a particular track, read the track into a cache slot(if the track is not already in cache), lock the cache slot, and thenreread the corresponding table entry.

Referring to FIG. 7, a flow chart 170 illustrates steps performed inconnection with writing information to a device table for a virtualdevice corresponding to a standard logical device or a log device.Processing begins at a first step 172 where it is determined if theparticular track corresponding to the device table entry being writtenis on the standard logical device or the log device. If it is determinedthe particular track of interest is on the standard logical device,control passes from the step 172 to a step 178 where the trackcorresponding to the device table entry being written is locked. Lockingthe track at the step 178 prevents other processes from getting accessto the track, and from modifying the corresponding table entry, whilethe current process is modifying the device table entry corresponding tothe track. Following the step 178 is a step 182 where the writeoperation is performed. Following the step 182 is a step 184 where thetrack is unlocked. Following the step 184, processing is complete.

If it is determined that the test step 172 that the track correspondingto the table entry for the virtual device that is being modified pointsto the log device, then control passes from the test step 172 to a step194 where the track of the log device corresponding to the entry of thedevice table that is being written is locked. Following the step 194 isa step 196 where the write operation is performed. Following the step196 is a step 198 where the track is unlocked. Following the step 198,processing is complete.

Referring to FIG. 8, a flow chart 210 illustrates steps performed inconnection with modifying a device table corresponding to a virtualdevice. This may be contrasted with the flow chart 170 of FIG. 7 thatillustrates modifying the device table for the standard logical deviceor the log device pointed to by an entry for a track of the device tablefor a virtual device. In flow chart 210, the device table for thevirtual device is modified, as opposed to the device table for thestandard logical device or the device table for the log device.

Processing begins at a first step 212 where it is determined if themodifications to the table relate to establishing the virtual device. Asdiscussed elsewhere herein, establishing a virtual device includesmaking the virtual device available for access by a host after thevirtual device is created. Establishing a virtual device causes thevirtual device to be associated with a standard logical device (andthus, represent a point in time copy of the standard logical device atthe time of establishment). Prior to being associated with a standardlogical device, a virtual device is not established and is notaccessible by a host. After being established, a virtual device isaccessible by a host.

If it is determined at the step 212 that the modifications to the tablerelate to establishing the virtual device, then control passes from thestep 212 to a step 214 where a device lock for the virtual device is setto prohibit access to the table by other processes. The device lock iscomparable to the cache slot lock, discussed elsewhere herein.

Following the step 214 is a step 216 where the pointers of the virtualdevice table are made to point to tracks of the standard logical deviceand where a protection bit is set for each of the tracks of the standardlogical device that corresponds to the virtual device being established.In an embodiment disclosed herein, each of the tracks of the standardlogical device has sixteen bits which may be set as protection bits, onefor each virtual device established to the standard logical device. Insome embodiments, the protection bits may have uses that are unrelatedto virtual devices. A new virtual device being established may beassigned a new bit position in the sixteen bit field while the bit foreach track of the standard logical device may be set. As discussed inmore detail elsewhere herein, the protection bit being set followed by asubsequent write to the standard logical device indicates that specialprocessing needs to take place to accommodate the virtual deviceestablished to the standard logical device. The special processing isdescribed in more detail elsewhere herein. Also at the step 216, thetrack entries for the device table for the virtual device are allmodified to point to the corresponding tracks of the standard logicaldevice. Thus, when the virtual device is first established, all of thepointers of the device table of the virtual device point to the tracksof the standard logical device.

Following the step 216 is a step 217 the virtual device is set to theready state, thus making the virtual device accessible to hosts.Following the step 217 is a step 218 where the virtual device isunlocked, thus allowing access by other processes. Following the step218, processing is complete.

If it is determined that the test step 212 that the virtual device isnot being established (i.e., some other operation is being performed),then control passes from the test step 212 to a step 222 to lock a trackcorresponding to the entry of the device table for the virtual devicethat is being modified. Note that the track that is locked at the step222 may either be a track on the standard logical device (if the entryof interest in the device table of the virtual device points to thestandard logical device) or a track of the log device (if the entry ofinterest points to the log device). Following the step 222 is a step 224where the modification to the device table for the virtual device isperformed. Following the step 224 is a step 226 where the track isunlocked. Following the step 226, processing is complete.

Referring to FIG. 9, a flow chart 230 illustrates steps performed inconnection with manipulating tracks of a log device. As discussed above,the tracks of a log device are maintained by creating a doubly linkedlist of tracks of the log device that are free (i.e. tracks that areavailable for accepting new data). Thus, if one or more tracks areneeded for use in connection with a corresponding virtual device, thefree tracks are obtained from the doubly linked list, which is modifiedin a conventional manner to indicate that the tracks provided for use bythe virtual device are no longer free. Conversely, if one or more tracksthat are used by one or more virtual devices are no longer needed, thetracks are returned to the doubly linked list, in a conventional manner,in order to indicate that the tracks are free. The flow chart 230 ofFIG. 9 illustrates the steps performed in connection with controllingaccess to the tracks (and track pointers) by multiple processes whichmanipulate the tracks.

Processing begins at a test step 232 where it is determined if theoperation being performed is modifying only tracks that are on the freelist. Note that modifying tracks only on the free lists by, for example,transferring a free track from one part of the list to another part orfrom one free lists to another free list (in the case of multiple freelists), does not involve modifications for tracks corresponding to anydata. If it is determined at the test step 232 that the modificationbeing performed does not involve only tracks on the free list, thencontrol transfers from the step 232 to a step 234 where the track islocked to prevent access by other processes.

Following the step 234 or the step 232 if the step 234 is not reached isa test step 236 where it is determined if the manipulation involves onlyallocated tracks. For any operation involving only allocated tracks, itis not necessary to lock the log device list of free tracks. If itdetermined at the step 236 that the operation being performed is notmanipulating only allocated tracks, then control transfers from the step236 to the step 238 where the log device list of free tracks is lockedto prevent access by other processes. Following the step 238, orfollowing the step 236 if the step 238 is not executed, is a step 242where the modification is performed.

Following the step 242 is a test step 244 where it is determined if themanipulation involves only allocated tracks. If it is determined at thetest step 244 that the modification being performed does not involveonly allocated tracks, then control transfers from the step 244 to astep 246 where the log device free list is unlocked. Following the step246 or the step 244 if the step 246 is not reached is a test step 248where it is determined if the operation being performed is modifyingonly tracks that are on the free list. If it determined at the step 248that the operation being performed is modifying only tracks that are onthe free list, then control transfers from the step 248 to the step 252where the track or tracks locked at the step 234 are unlocked. Followingthe step 252, or following the step 248 if the step 252 is not executed,processing is complete.

Referring to FIG. 10, a flow chart 280 illustrates steps performed inconnection with reading data from a virtual device. Processing begins ata test step 282, where it is determined if the device table entry forthe track of interest of the virtual device points to the standardlogical device or points to the log device. If it is determined at thetest step 282 that the table points to the standard logical device, thencontrol passes from the step 282 to a step 284, where the track is readfrom the standard logical device. Following the step 284, processing iscomplete. Alternatively, if it determined at the test step 282 that thedevice table of the virtual device points to the log device, thencontrol passes from the step 282 to a step 286, where the track ofinterest is read from the log device. Following the step 286, processingis complete.

Note that in some instances, it may be possible that prior to the teststep 282, it is determined that the track of interest being read isalready in the cache memory (global memory). In that case, the track maybe obtained from the cache memory without executing any of the steps282, 284, 286.

Referring to FIG. 11, a flow chart 300 illustrates steps performed by aDA in connection with writing to a track of a standard logical device towhich a virtual device has been previously established. Processingbegins at a first step 302 where it is determined if any protection bitsfor the track being written on the standard logical device have beenset. If it determined at the test step 302 that the protection bits arenot set, then control transfers from the step 302 to a step 304, where anormal write operation is performed. That is, at the step 304, data iswritten to the standard logical device in a conventional fashion withoutregard to the existence of a virtual device that had been previouslyestablished to the standard logical device. Following the step 304,processing is complete.

If it is determined at the test step 302 that one or more protectionbits have been set on the track of the standard logical device that isbeing written, control passes from the step 302 to a step 306, where afree track of the log device is obtained. The free track of the logdevice is needed to copy data from the track of the standard logicaldevice. Also, as described in more detail elsewhere herein, free tracksof the log device may be managed using a doubly-linked list of the freetracks. Thus, at the step 306, it may be possible to obtain a free trackby traversing the list of free tracks of the log device and modifyingthe pointers appropriately to remove one of the free tracks for use.

Following the step 306 is a step 308, where, for each virtual devicethat corresponds to a protection bit that was determined to be set atthe test step 302, the pointers of the virtual devices, which initiallypointed to the track being written on the standard logical device, aremodified at the step 308 to point to the free track of the log deviceobtained at the step 306. As discussed above, it is possible to havemore than one virtual device established to a standard logical device.For each virtual device that has been established to a particularstandard logical device, a specific protection bit will be set for eachof the tracks of the standard logical device. Thus, at the step 308, thetrack pointers are changed for all the virtual devices corresponding toa set protection bit detected at the step 302. The track pointers in thedevice tables of virtual devices are modified to point to the new trackthat was obtained at the step 306.

Following the step 308 is a step 312, where the data is caused to becopied from the standard logical device to the new track on the logdevice that was obtained at the step 306. In an embodiment disclosedherein, the data may be copied by moving the data from disk storage tothe global memory of the storage device (e.g., into a cache slot), andthen setting a write pending indicator to cause the data to be copied tothe track of the log device obtained at the step 306. The step 312represents copying the data from the track of the standard logicaldevice that is being written to the new track of the log device obtainedat the step 306. Since all the pointers are modified at the step 308,any virtual device that has been established to the standard logicaldevice prior to the track being written now points to the old data(i.e., the data as it existed on the track of the standard logicaldevice when the virtual devices were established). Note also that, inconnection with copying the track, the protection bits of the standardlogical device track are copied to virtual device map bits for the trackon the log device, which is explained in more detail elsewhere herein.

Following the step 312 is a step 314, where the track of the log deviceobtained at the step 306 is modified so that the extra bytes in thetable (discussed elsewhere herein) are made to point back to the trackof the standard logical device that is being written. Having the trackof the log device point to the corresponding track of the standardlogical device from which the data was provided is useful in manyinstances. For example, it may be useful in connection with datarecovery. Following the step 314 is a step 316, where the protectionbits of the tracks of the standard logical device being written arecleared. Following the step 316 is a step 318, where status is sent tothe HA. Following the step 318, processing is complete.

Note that once the HA receives status, the HA may perform a normal writeoperation and, in that case, at the test step 302, the protection bitswill not be set, since the bits are cleared at the step 316. The HA thatis performing the write operation sees the protection bits that are setat the step 302 and sends a protection request to the appropriate DA.The HA then may disconnect from the DA and wait for status to arrivefrom the DA indicating that a normal write may be performed. While theHA is disconnected and waiting for status from the DA, the DA mayperform the steps disclosed in the flow chart 300. This is described inmore detail below.

Referring to FIG. 12, a flow chart 320 illustrates steps performed by anHA in connection with a write to a standard logical device to which oneor more virtual devices have been established. Processing begins at afirst test step 322, where it is determined if any protection bits areset for the tracks of the standard logical device that are beingwritten. If it is determined at the test step 322 that no protectionbits are set, then control passes from the step 322 to a step 324, wherea normal write is performed. Following the step 324, processing iscomplete.

If it is determined at the test step 322 that one or more protectionbits are set for the tracks of the standard logical device that arebeing written, control passes from the step 322 to a step 326, where theHA sends a request to the DA indicating that protection bits are set forthe tracks. When the DA receives the request that is sent at the step326, the DA performs the operations set forth in the flow chart 300 ofFIG. 11, discussed above. Following the step 326 is a step 328, wherethe HA disconnects from the DA in order to allow (possibly unrelated)operations to be performed with the DA by other processes and/or otherHA's.

Following the step 328 is a step 332, where the HA waits for the DA toperform the operations set forth in the flow chart 300 of FIG. 11 and tosend status to the HA indicating that the appropriate steps have beenperformed to handle the set protection bits. Following the step 332,processing transfers back to the step 322, where the protection bits forthe track of the standard logical device are again tested. Note that ona second iteration, it is expected that the protection bits of the trackof the standard logical device that are being written would be clear atthe step 322, since the DA would have cleared the protection bits inconnection with performing the steps of the flow chart 300. Of course,it is always possible that a new virtual device will be established tothe standard logical device in between the DA clearing the protectionbits and the step 322 being executed again. However, it is usuallyexpected that the second iteration of the step 322 for a particulartrack of the standard logical device will determine that all theprotection bits are clear, and control will transfer from the step 322to the step 324 to perform a normal write.

Referring to FIG. 13, a flow chart 340 illustrates steps performed inconnection with writing to a virtual device. The flow chart 340represents steps performed by both the HA and the DA and thus could havebeen provided as two flow charts, similar to the flow chart 300 of FIG.11 and the flow chart 320 of FIG. 12. However, it will be understood bythose of ordinary skill in the art that the flow chart 340 may representa division of steps similar to those set forth in the flow charts 300,320 and described in the corresponding portions of the text of thespecification.

Processing begins at a first step 342, where it is determined if thevirtual device points to the standard logical device. If so, thencontrol transfers from the test step 342 to a step 344, where a freetrack of the log device is obtained. Following the step 344 is a step346, where data from the standard logical device corresponding to thetrack being written is caused to be copied from the standard logicaldevice to the track of the log device obtained at the step 344.Following the step 346 is a step 348, where the virtual device pointerfor the track is adjusted to point to the track obtained at the step344. Following the step 348 is a step 352, where a protection bitcorresponding to the virtual device is cleared in the track data of thestandard logical device, thus indicating that no special processing onbehalf of the virtual device is required when writing to the track ofthe standard logical device. Following the step 352 is a step 354, wherethe write is executed. At the step 354, the data to be written may be atrack or a portion of a track that is written to the track obtained atthe step 344. Following the step 354, processing is complete. If thedata corresponds to an entire track, then it may be possible toeliminate the step 346, which copies data from the track of the standardlogical device to the new track of the log device, since writing anentire track's worth of data at the step 354 would overwrite all of thedata copied at the step 346.

If it is determined at the test step 342 that the pointer for the trackof the virtual devices being written does not point to the standardlogical device, then control transfers from the step 342 to a test step356, where it is determined if more than one virtual devices have beenestablished to the standard logical device. If not, then controltransfers from the step 356 to a step 358, where a normal writeoperation to the track of the log device is performed. If it isdetermined at the test step 356 that there is more than one virtualdevice established to the standard logical device, then controltransfers from the step 356 to a step 362, where a free track from thelog device is obtained.

Following the step 362 is a step 364, where the data of the trackcorresponding to the virtual device being written is copied to the trackobtained at the step 362. Following the step 364 is a step 366, wherethe virtual device pointers are adjusted to point to the new track. Inone embodiment, the pointer for the virtual device that is being writtenis made to point to the new track. Alternatively, it is possible to notchange the pointer for the virtual device that is being written and,instead, adjust all the pointers for all of the other virtual devicesthat point to the track at the step 366.

Following the step 366 is a step 368 where the virtual device map bitsfor the tracks of the log device are modified. For the log devicetracks, the virtual device map bits may be used to indicate whichvirtual devices point to each track, where, in one embodiment, there aresixteen virtual device map bits and each bit corresponds to a particularvirtual device. Thus, the test at the step 356 may examine the virtualdevice map bits for the track.

Following the step 368 is a step 369, where the write is executed. Notethat whether the write is executed to the track obtained at the step 362or to the track that is initially pointed to by the virtual device beingwritten depends upon how the pointers are adjusted at the step 366. Inall cases, however, data is written to the track pointed to by thevirtual device to which the data is being written. Following the step369, processing is complete.

Referring to FIG. 14, a flow chart 370 illustrates steps performed inconnection with removing (i.e., eliminating) a virtual device. Once avirtual device has been established and used for its intended purpose,it may be desirable to remove the virtual device. Processing begins at afirst step 372, where a pointer is set to point to the first track ofthe virtual device. The virtual device is removed by examining eachtrack corresponding to the virtual device.

Following the step 372 is a step 374, where it is determined if thetrack of the virtual device that is being examined points to thestandard logical device. If so, then control transfers from the step 374to a step 376 to clear the protection bit on the track of the standardlogical device corresponding to the virtual device being removed.Following the step 376 is a step 378, where a pointer points to the nexttrack of the virtual device in order to continue processing by examiningthe next track. Following the step 378 is a step 382, where it isdetermined if processing complete (i.e., all the tracks of the virtualdevice have been processed). If not, then control transfers from thestep 382 back to the test step 374, discussed above.

If it is determined at the test step 374 that the track of the virtualdevice being examined does not point to the standard logical device,then control transfers from the step 374 to a step 384, where a virtualdevice map bit on the track of the log device that corresponds to thevirtual device being removed is cleared. Each track of the log devicemay have a set of virtual device map bits indicating which virtualdevices use the track of the log device. Thus, at the step 384, thevirtual device map bit corresponding to the virtual device being removedis cleared.

Following the step 384 is a test step 386, where it is determined if thebit that was cleared at the step 384 was the last virtual device map bitthat was set for the track. In other words, the test step 386 determinesif there are other virtual devices that are using the track on the logdevice. If it is determined at the test step 386 that the last virtualdevice map bit was cleared at the step 384 (and thus, no other virtualdevices use the track), then control transfers from the step 386 to astep 388, where the track of the log device is returned to the free listof tracks of the log device, discussed elsewhere herein. Following thestep 388, or following the step 386 if it is determined that the bitcleared at the step 384 is not the last virtual device map bit of thetrack of the log device, is the step 378, discussed above, where thenext track of the virtual device is pointed to for subsequentexamination. Once all of the tracks corresponding to the virtual devicehave been processed, the tables and other data structures associatedwith the virtual device may also be removed although, in someembodiments, the tables and other data structures from the virtualdevice may be maintained, so long as the virtual device is not madeavailable for use by hosts after the virtual device is deestablished.

In some embodiments, the virtual device may be made not ready to hostsprior to performing the steps illustrated by the flow chart 370.Alternatively, it may be possible to perform the steps illustrated bythe flow chart 370 while the virtual device is ready to hosts and tosimply issue an error message to any host that attempts to access thevirtual device while the steps of the flow chart 370 are being performedand/or to any host attempting to access a track of the virtual devicethat has already been destroyed (i.e., returned to the list of freetracks).

Any one of a variety of techniques may be used for setting theprotection bits on the tracks of the standard logical device at the step216 of FIG. 8. For example, the protection bits for the tracks of thestandard logical device may be set by locking the entire standardlogical device (thus prohibiting any other access to the standardlogical device) while all of the protection bits are being set. However,locking the entire standard logical device while all of the protectionbits are being set may be unacceptable in certain instances.Accordingly, other techniques, described below, are available to set theprotection bits in a way that does not necessarily cause the standardlogical device to be inaccessible for as long as it takes to set all ofthe protection bits for all of the tracks.

Referring to FIGS. 15A, 15B, and 15C, flow charts 400, 410, 420illustrates steps performed in connection with a technique for settingprotection bits of a standard logical device. The flow charts 400, 410,420 are shown separately to illustrate that different parts of theprocess may be performed separately.

In the first flow chart 400 of FIG. 15A, processing begins at a firststep 404, where a session number is obtained to reserve a particularsession. In some embodiments, reserving a session number may be referredto as “attaching a session”. In an embodiment disclosed herein, eachparticular session number corresponds to a particular bit position of aprotection bit mask (i.e., corresponds to a particular protection bit).Following the step 404, processing is complete. In some embodimentswhere multiple processes may attempt to obtain a session numbersimultaneously, it may be necessary to provide additional functionalityso that no two processes obtain the same session number. Thisfunctionality may be provided, for example, by locking the sessionresources (using software locks, hardware locks, or both) prior toobtaining the session number and then unlocking after obtaining thesession number. Of course, other techniques exist and may be used toprovide that the step 404 returns a unique session number to any processthat requests a session number, even if multiple processes arerequesting a session number at the same time.

Referring to FIG. 15B, the flow chart 410 illustrates steps forestablishing a session to an inactive state and setting the protectionbits. Processing begins at a first step 412, where the calling processis provided with exclusive access to the standard logical device (or atleast to the portions the standard logical device relating to theoperations that follow). In some instances, it is useful to provide acalling process with exclusive access to the resources being manipulatedbecause simultaneous (or near simultaneous) accesses by multipleprocesses could cause improper operation. Thus, at the step 412 and inother instances throughout this application, exclusive access is givento a calling process. In addition, the mechanism for providing exclusiveaccess could include any one of a variety of techniques, such ashardware locks, software locks (e.g., of system level data), timeslicing, etc. Of course, in embodiments where there can only be onepossible calling process (e.g., a non-multitasking system), it may notbe necessary to perform any processing like that illustrated in the step412 to enable exclusive access to a calling process.

Following the step 412 is a step 414 where the session (reservedpreviously at the step 404, discussed above) is established to aninactive state. In an embodiment herein, establishing the session to aninactive state involves moving a first value to a location in the headerof the device table for the standard logical device. As discussed above,FIG. 5 shows the header field 114 in the device table 112. The headerfield 114 can contain various data locations, each of which correspondsto one of the protection bits (i.e., where each bit position correspondsto a particular session number). A value placed at each of the locationsindicates the operation to be performed when a write occurs to a trackhaving the corresponding one of the protection bits set. For example,there may be sixteen byte-length data locations in the header field 114,where each data location corresponds to one of sixteen possibleprotection bits (e.g., byte zero corresponds to protection bit zero,byte one corresponds to protection bit one, etc.). At the step 414, afirst value is provided to one of the locations of the header field 114to set the corresponding session to an inactive state.

Setting the session to an inactive state at the step 414 causes nooperations to be performed when the corresponding protection bit is set.Thus, when a write occurs to a track where the protection bit is set,the code that handles management of protection bits will fetch thecorresponding data from the device header, which in this case willindicate that the corresponding session is in an inactive state. Inresponse to this, the code that handles management of protection bitswill leave the protection bit set and will perform no other operations.The utility of this is discussed elsewhere herein.

Following the step 414 is a step 416 where the exclusive access providedat the step 412 is disabled. Disabling exclusive access at the step 416allows multiple processes simultaneous access to the resources to whichexclusive access was provided at the step 412. In some embodiments,disabling exclusive access at the step 416 simply undoes whatever wasdone at the step 412 (e.g., unlocking locked resources).

Following the step 416 is a step 418 where the corresponding protectionbit is set for each of the tracks corresponding to the location of theheader field 114 for the session number that was obtained at the step404. Note that access by multiple process is provided to the standardlogical device while the step 418 is performed. However, since thecorresponding session is inactive, then any writes to tracks having abit set at the step 418 that occur while the step 418 is being performedwill result in no operations being performed and the protection bitremaining set. Following step 418, processing is complete. Note that thestep 418 of setting the protection bits may be performed any time afterthe session is made inactive (and before the session is made active,discussed below). Thus, the step 418 does not necessarily need toimmediately follow the steps performed to make the session inactive.

In some embodiments, it may not be necessary to enable/disable exclusiveaccess to the resources of the standard logical device. For example, ifestablishing the session to an inactive state at the step 414 may beperformed in a single unitary step (e.g., one uninterruptible writeoperation), it may not be necessary to execute the steps 412, 416. Thisis illustrated by alternative paths 419 a, 419 b shown in the flow chart410.

Referring to FIG. 15C, the flow chart 420 illustrates steps performed toactivate a session. Processing begins at a first step 422 whereexclusive access to the standard logical device is enabled (like in thestep 412, discussed above), thus preventing access thereto by anotherprocess. Following the step 422 is a step 424 where the session is madeactive by, for example, writing a second value to the header field 114of the table for the standard logical device. The second value isprovided in the same location as the first value and overwrites thefirst value. The second value indicates the special processing that isto be performed in connection with the corresponding protection bitbeing set. Such special processing is shown, for example, in FIGS. 11and 12 which illustrate special processing for virtual devices.

Following the step 424 is a step 426 where exclusive access to thestandard logical device is disabled. Following the step 426, anysubsequent writes to tracks having a set protection bit will cause thespecial operation to be performed as indicated by the second valueprovided to the header field 114 of the device table for the standardlogical device. For example, if the second value provided at the step424 indicates that the corresponding track should be copied to a logdevice, that is the operation that will be performed in connection witha write to a track with a set protection bit. Of course, the secondvalue provided at the step 424 can indicate any one of a number ofspecial processes to be performed in connection with the protection bitfor the track being set, such as, for example, processing to beperformed in connection with a snap operation. Following the step 426,processing is complete.

Just as with the flow chart 410, discussed above, in some embodiments,it may not be necessary to enable/disable exclusive access to theresources of the standard logical device used in connection with thestep 424. For example, if establishing the session to an active state atthe step 424 may be performed in a single unitary step (e.g., one writeoperation), it may not be necessary to execute the steps 422, 426. Thisis illustrated by alternative paths 428 a, 428 b shown in the flow chart420.

Referring to FIG. 16A, a portion of a flow chart 300′ shows steps thatcorrespond to steps of the flow chart 300 of FIG. 11. The steps of theflow chart 300′ are modified to account for the use of the protectionbit scheme illustrated in FIGS. 15A, 15B, and 15C and described above. Apair of steps 302′, 302″ replace the step 302 of the flow chart 300 ofFIG. 11. The step 302′ is a test step like the step 302 where it isdetermined if the protection bit for the track being written to is set.If not, then control passes from the step 302′ to the step 304,discussed above in connection with FIG. 11, where a normal write isperformed. Following the step 304, processing is complete.

If it is determined at the test step 302′ that the protection bit of thetrack being written to is set, control passes from the step 302′ to thetest step 302″ where it is determined if the corresponding session isinactive. As discussed above in connection with FIGS. 15A, 15B, and 15C,a session may be inactive so that no operations are performed inresponse to the protection bit being set. If it is determined at thetest step 302″ that the session is inactive, then control passes fromthe step 302″ to the step 304, discussed above. Otherwise, controlpasses from the step 302″ to continue on processing at the step 306,discussed above in connection with FIG. 11.

Referring to FIG. 16B, a portion of a flow chart 320′ shows steps thatcorrespond to steps of the flow chart 320 of FIG. 12 that are modifiedto account for the use of the protection bit scheme illustrated in FIGS.15A, 15B, and 15C and described above. A pair of steps 322′, 322″replace the step 322 of the flow chart 320 of FIG. 12. The step 322′ isa test step like the step 322 where it is determined if the protectionbit for the track being written to is set. If not, then control passesfrom the step 322′ to the step 324, discussed above in connection withFIG. 12, where a normal write is performed. Following the step 324,processing is complete.

If it is determined at the test step 322′ that the protection bit of thetrack being written to is set, control passes from the step 322′ to thetest step 322″ where it is determined if the corresponding session isinactive. If it is determined at the test step 322″ that thecorresponding session is inactive, then control passes from the step322″ to the step 324, discussed above. Otherwise, control passes fromthe step 322″ to continue on processing at the step 326, discussed abovein connection with FIG. 12.

Establishing a virtual device to a standard logical device may beperformed using three separate system calls. The first, Register,reserves a session number and corresponding bit position in theprotection bits of the standard logical device and, for someembodiments, creates or obtains a corresponding virtual device. Thesecond, Relate, relates a virtual device with the standard logicaldevice by modifying the pointers for the virtual device, as describedabove, and also sets the protection bits of the standard logical device.The third, Activate, causes the virtual device to represent a point intime copy at the time that Activate is invoked and, in some cases, makesthe virtual device ready to a host. In the case of using three systemcalls, the steps 214, 216, 217, 218 of FIG. 8, discussed above, may notbe performed or may be performed differently, as set forth in thediscussion below.

In addition, as described in more detail below, it may be possibleperform the Register and Relate steps for multiple pairs of virtualdevices and standard logical devices and then perform a single Activatestep that causes all of the virtual devices to be established to theircorresponding standard logical devices. Note also that, for purposes ofthe description herein, standard logical device may refer to any logicalstorage device generally having its own storage tracks (even if some ofthe tracks could be indirect at times) while virtual storage device mayrefer to a storage device that, by definition, uses storage tracks ofother devices.

Referring to FIG. 17, a flow chart 430 illustrates steps performed inconnection with registering a standard logical device. Registering maybe performed using a system call (syscall) that is passed identifiersfor the standard logical device and, for some embodiments, acorresponding virtual device. In some embodiments, the Register syscallmay be passed only the standard logical device and may return anidentifier for a virtual device created by the Register syscall. Inother embodiments, the Register syscall does not handle any virtualdevices and is simply passed a standard logical device.

Processing begins at a first step 432 where a session number is reservedin a manner similar to that discussed above in connection with the step404 of FIG. 15A. Following step 432 is a step 433 where exclusive accessto appropriate resources (e.g., of the standard logical device) isenabled in a manner similar to that discussed above in connection withFIGS. 15B and 15C. Following the step 433 is a step 434 where thecorresponding session is established to an inactive state in a mannersimilar to that discussed above in connection with the step 414 of FIG.15B. Following the step 434 is a step 436 where exclusive access toappropriate resources is disabled in a manner similar to that discussedabove. Note that establishing the session to an inactive state does notalter write processing to the standard logical device. Note also that,just as with FIGS. 15B and 15C, for some embodiments it may not benecessary to enable and disable exclusive access to the resources usedin connection with the step 434. This is illustrated by alternativepaths 437 a, 437 b.

Following the step 436 is a test step 438 which determines if theoperations performed at the previous steps 432-434, 436 were successful.The operations may not have been successful for a variety of reasonsincluding, for example, the fact that all of the protection bits for thestandard logical device have already been used for other purposes. In anembodiment illustrated herein, there are sixteen protection bits. Thus,if prior to executing step 432, all sixteen protection bits for thestandard logical device are being used, then the result at the test step438 will indicate that the previous operations were not successful. Ifit is determined at the test step 438 that the previous operations werenot successful, control passes from the step 438 to a step 442 where anerror is returned. Following step 442, processing is complete.

If it is determined at the test step 438 that the previous operationswere successful, then control passes from the step 438 to a step 444where the virtual device is created. In some embodiments, the virtualdevice is created at the step 444 and passed back to the caller of theRegister routine. In other embodiments, the virtual device may existprior to invoking the Register routine, in which case there is no needto create a new virtual device at the step 444. In still otherembodiments (discussed below), the Register routine does not handle(i.e., create or get passed) any virtual devices. Rather, the virtualdevice may be created or obtained in a separate step either before orafter the Register routine, such as in connection with a Relate routine(described below), or as part of a general system configuration.

Following step 444 is a step 446 where information regarding thestandard logical device (and perhaps a virtual device, if one is createdand/or used in connection with the Register routine) is placed in theheader of the device table for the standard logical device. Providingthe information in the header of the device table of the standardlogical device facilitates processing later on once the standard logicaldevice and a corresponding virtual device is activated (describedbelow). In some embodiments, it may be useful to enable exclusive accessto appropriate resources of the standard logical device prior to placingthe information in the header. Following the step 446, processing iscomplete.

Note that the processing illustrated in the flow chart 430 does notcause any virtual device to be a point in time copy of the standardlogical device and does not make any virtual device accessible. TheRegister routine illustrated by the flow chart 430 corresponds topreliminary operations that facilitate later activation of a standardlogical device/virtual device pair. Note also an alternative path 448from the step 438 to the step 446 illustrates embodiments where theRegister routine does not cause any virtual device to be created. Thus,if the Register routine does not handle virtual devices, then controlpasses from the test step 438 directly to the step 446 via the path 448if the Register operation was successful.

Referring to FIG. 18, a flow chart 450 illustrates steps performed inconnection with relating a virtual device to a standard logical device.Just with the Register routine, the Relate routine is part of thepreliminary processing that is performed prior to activation and use ofa standard logical device/virtual device pair. Relating may be performedby making a system call in which the parameters are the standard logicaldevice and, for some embodiments, a corresponding virtual device(passed, for example, by a host). In other embodiments, the Relateroutine may cause a virtual device to be created (e.g., by calling aseparate create routine) or may simply obtain a preexisting unusedvirtual device that is provided in connection with system configurationor provided by some other means. In those embodiments, the Relateroutine may be passed a pointer to storage that the Relate routine usesto place an identifier for the newly created/obtained virtual device.

Processing begins at a first test step 452 where it is determined if thestandard logical device being related has been previously registered.Note that the Relate routine may not be called for a standard logicaldevice unless the standard logical device has been previouslyregistered. If the standard logical device has not been registered, thencontrol passes from the step 452 to a step 454 wherein an error isreturned. Following the step 454, processing is complete. Note that thestep 454 (and other error steps described herein) may actually refer toseparate error processing that does something different than reporterrors. For example, the processing performed at the step 454 mayinclude measure taken to correct an error and continue processing.

If it is determined that the test step 452 that the standard logicaldevice has been registered, then control passes from the step 452 to astep 455 where a virtual device is obtained for pairing with thestandard logical device. In some embodiments, the virtual device iscreated or obtained in connection with the Register routine (or at leastbefore the Relate routine is called), in which case the step 455 mayrepresent the Relate routine being passed the virtual device previouslycreated (by, for example, a host) or obtained and an identifier for thevirtual device is passed back to the calling routine. In otherembodiments, the virtual device is created or obtained by the Relateroutine at the step 455. Following the step 455 is a step 456 whereexclusive access to appropriate resources (e.g., the standard logicaldevice) is enabled in a manner similar to that discussed above inconnection with FIGS. 15B and 15C. Following the step 456 is a step 458where all the pointers in the device table for the virtual device aremade to point to corresponding tracks of the standard logical device.The processing performed at the step 458 is analogous to the pointermodifications provided at the step 216 of the flow chart 210 of FIG. 8,discussed above. Following the step 458 is a step 461 where informationabout the standard logical device/virtual device pair is placed in theheaders of both the standard logical device and the virtual device. Thisinformation is used in connection with subsequent accesses for thedevices, as described elsewhere herein. Following the step 461 is a step462 where exclusive access to appropriate resources is disabled in amanner similar to that discussed above. In some embodiments, exclusiveaccess may be enabled prior to placing the information in the header andmay be disabled after the information is placed therein. Note that, justas with FIGS. 15B and 15C, for some embodiments it may not be necessaryto enable and disable exclusive access to the resources used inconnection with the step 458 and/or the step 461. This is illustrated byalternative paths 463 a, 463 b.

Following the step 462 is a step 464 where the protection bits for thestandard logical device are set, thus indicating special processing tobe performed when a write is provided to a track of the standard logicaldevice. Note, however, that when the protection bits are set at the step464, no special processing will take place upon writes to the tracks ofthe standard logical device because, as discussed above, the session isinitially inactive. Thus, even though a protection bit may have been setfor a track at the step 464, writes to the standard logical device willcause no special processing is to be performed because the session isinactive. This is illustrated above in connection with the steps 302″and 322″ of FIG. 16A and FIG. 16B.

Following the step 464 is a step 466 where the standard logicaldevice/virtual device pair is added to a list. As discussed in moredetail below, it is possible to activate a plurality of standard logicaldevice/virtual device pairs with one call. The list used at the step 466contains a list related pairs that have not yet been activated. Asdiscussed in more detail below, the list is used by the Activationroutine. Alternatively, the Relate routine may return the standardlogical device/virtual device pair (or just the virtual device or justthe standard logical device) to the calling routine (e.g., a hostapplication) that maintains the list. Alternatively still, the Relateroutine may be passed a list identifier, which is used by the Relateroutine to determine a particular list to which the standard logicaldevice/virtual device pair is to be added. In that case, it may beuseful to have a separate routine that creates the list identifiersand/or list structures/storage and which maintains the lists. Ininstances where the list and/or list id is a passed parameter, it may bepossible for the calling routine to maintain more than one list. TheRelate routine may create the list after being passed a plurality ofstandard logical devices and, optionally, a corresponding plurality ofvirtual devices. In that case, the Relate routine may pass back the listthat is created.

Referring to FIG. 19A, a flow chart 480 illustrates steps performed inconnection with the Activate routine that activates one or more standardlogical device/virtual device pairs. Prior to activation, an unusedvirtual device is not ready to any host. However, in the course ofactivation, a virtual device is made ready to one or more hosts.

Processing begins at a first step 482 where a pointer is set to point tothe first item on a list like the list described above in connectionwith the step 466 of the flow chart 450. The pointer is used to point tovarious elements on the list. The list may be passed as a parameter tothe Activate routine. Alternatively, a list id may be passed where thelist id is used by the Activate routine to distinguish between lists towhich the Activate routine has access. Alternatively still, the Relateand Activate routines may use a single global list so that any call tothe Activate routine causes all previously-related standard logicaldevice/virtual device pairs to be activated. Of course, instead of aformal list structure, it may be possible to pass each of the standardlogical device/virtual device pairs as parameters to the Activateroutine.

Following the step 482 is a test step 484 where it is determined ifprocessing is complete (i.e. if the end of the list has been reached).This may be determined by examining the pointer used to iterate throughthe list. If processing of the list is not complete, then control passesfrom the step 484 to a step 485, which determines if the Relateoperation was previously performed on the standard logicaldevice/virtual device pair indicated by the pointer. If the Relateoperation was not previously performed (and thus no activation ispossible), then control transfers from the step 485 to a step 486 wherean error is returned. Following the step 486, processing is complete.

If it is determined at the step 485 that the Relate routine wasperformed on the standard logical device/virtual device pair beingactivated, then control transfers from the step 485 to a step 487 whereexclusive access to appropriate resources (e.g., the standard logicaldevice) is enabled in a manner similar to that discussed above inconnection with FIGS. 15B and 15C. Following the step 487 is a step 488where the pointer is made to point to the next standard logicaldevice/virtual device pair on the list. Following the step 488, controltransfers back to the test step 484 to determine if processing of thelist is complete.

If it is determined at the test step 484 that processing of the list iscomplete (and thus all standard logical devices corresponding to virtualdevices to be activated have been determined to have been previouslyrelated), control passes from the step 484 to a step 492 where thepointer that keeps track of elements on the list is made to point to thefirst item in the list of standard logical device/virtual device pairs.Following the step 492 is a test step 494 where it is determined if theentire list has been processed. If not, then control transfers from thestep 494 to a step 496 to activate the session corresponding to theprotection bit that has been set for the standard logical device/virtualdevice pair. Activating the session at the step 496 is analogous to theprocessing performed at the step 424 in the flow chart 420, of FIG. 15C,discussed above. Activating the session at the step 496 causes thespecial virtual device processing, discussed elsewhere herein, to beperformed when a write occurs to a track of the standard logical devicehaving a set protection bit. In addition, activating a session makes thevirtual device ready to a host. Following the step 496 is a step 498where the pointer that is keeping track of the processed items on thelist is made to point to the next item. Following the step 498, controltransfers back to the test step 494.

If it is determined at the test step 494 that the entire list ofstandard logical device/virtual device pairs has been processed (so thatall standard logical devices corresponding to virtual devices beingactivated have had the corresponding sessions made active), controltransfers from the step 494 to a step 502 where the pointer is made topoint to the first standard logical device/virtual device pair in thelist. Following the step 502 is a step 504 where it is determined if theentire list has been processed. If is determined that the entire listhas been processed, control transfers from the step 504 to a step 506where exclusive access to appropriate resources (obtained at the step487) is disabled in a manner similar to that discussed above. Followingthe step 506 is a step 508 where the pointer is made to point to thenext item on the list. Following the step 508, control transfers back tothe test step 504 to determine if the end of the list has been reached.

If it is determined at the test step 504 that the end of the list hasbeen reached, control transfers from the step 504 to a step 512 wherethe list is cleared so that a subsequent call to the Activate routinewill not attempt to reactivate already activated standard logicaldevice/virtual device pairs. Following the step 512, processing iscomplete. Note that, just as with FIGS. 15B and 15C, for someembodiments it may not be necessary to enable and disable exclusiveaccess to the resources used in connection with the steps 487, 506. Thisis illustrated by alternative paths 513 a, 513 b.

Note that, in some instances, exclusive access for some standard logicaldevices of a list may have been enabled at the step 487 prior to one ofthe devices on the list causing an error at the step 485. In thosecases, prior to execution of the step 486, the standard logical devicesto which exclusive access has already been enabled have exclusive accessthereto disabled in a manner similar to that illustrated above inconnection with the steps 502, 504, 506, 508.

In some embodiments, it may be possible to not use a list at the step466 of FIG. 18 for keeping track of the standard logical device/virtualdevice pairs. Instead, the Activate routine could be called by passingthe various standard logical device/virtual device pairs directlythereto. In that case, the list used in the processing illustrated bythe flow chart 480 of FIG. 19A would be the list of parameters passed tothe Activate routine.

In other embodiments, it may be possible to provide additional callssuch as Begin Group and Process Group where a Begin Group call isprovided prior to a plurality of Relate calls (or Register and Relatecalls). Then, when it is time to activate the various standard logicaldevice/virtual device pairs that were registered and related after theBegin Group call, the call to Process Group is made. The Process Groupcall, in effect, activates all of the standard logical device/virtualdevice pairs that were registered and related after the Begin Groupcall. Such embodiments may use the list, discussed above in connectionwith FIGS. 18 and 19A, where a Begin Group call causes creation of anew, empty, list and a Process Group call causes the processing shown inthe flow chart 480 of FIG. 19A to be performed. In such a system, aRelate call that is not bracketed by a Begin Group/Process Group pair(i.e. a Relate that is not called after a Begin Group call) could causethe Activate to be executed immediately after the relate is successfullycompleted. That is, any Register call and Relate call for a standardlogical device/virtual device pair that is not after a Begin Group callmay cause automatic activation of the standard logical device/virtualdevice pair. In other embodiments, the Activate call would not beautomatic. Note also that, in some embodiments, it may be possible touse multiple lists to separately activate different sets of standardlogical device/virtual device pairs. In those cases, the specific listmay be a parameter passed to a Process Group call, which could then bepassed on, in some fashion, to the Activate routine.

In some embodiments, any one of the Register, Relate and/or Activatecalls may include optional parameters for modifying the device name (orother device identifiers) for the virtual device. This may be useful inoperating systems where it is impermissible to have two devices thathave exactly the same name and/or device identifiers. If a virtualdevice is a copy of a standard logical device, it may be necessary tochange the name of the virtual device. In addition, in some embodiments,it may be possible to have an optional parameter indicating whether thevirtual device will be on line or off line upon activation. In someoperating systems, an on-line device is accessible to a host (ready tothe host) while an off-line device (not ready to a host) is not. Thus,for host applications that wish to create a virtual device but do notwish to permit access thereto, it may be possible to pass a parameter toany one of the Register, Relate and/or Activate calls to indicate thatthe virtual device is to be on line or off line upon activation.

Referring to FIG. 19B, a flow chart 480′ illustrates steps performed inconnection with an alternative embodiment for the Activate routine thatactivates one or more standard logical device/virtual device pairs.Processing begins at a first step 482′ where a pointer is set to pointto the first item on a list like the list described above in connectionwith the step 466 of the flow chart 450. The pointer is used to point tovarious elements on the list. The list may be passed as a parameter tothe Activate routine. Alternatively, a list id may be passed where thelist id is used by the Activate routine to distinguish between lists towhich the Activate routine has access. Alternatively still, the Relateand Activate routines may use a single global list so that any call tothe Activate routine causes all previously-related standard logicaldevice/virtual device pairs to be activated. Of course, instead of aformal list structure, it may be possible to pass each of the standardlogical device/virtual device pairs (or only one pair) as parameters tothe Activate routine.

Following the step 482′ is a test step 484′ where it is determined ifprocessing is complete (i.e. if the end of the list has been reached).This may be determined by examining the pointer used to iterate throughthe list. If processing of the list is not complete, then control passesfrom the step 484′ to a step 485′, which determines if the Relateoperation was previously performed on the standard logicaldevice/virtual device pair indicated by the pointer. If the Relateoperation was not previously performed (and thus no activation ispossible), then control transfers from the step 485′ to a step 486′where an error is returned. Following the step 486′, processing iscomplete.

If it is determined at the step 485′ that the Relate routine wasperformed on the standard logical device/virtual device pair beingactivated, then control transfers from the step 485′ to a step 488′where the pointer is made to point to the next standard logicaldevice/virtual device pair on the list. Following the step 488′, controltransfers back to the test step 484′ to determine if processing of thelist is complete.

If it is determined at the test step 484′ that processing of the list iscomplete (and thus all standard logical devices corresponding to virtualdevices to be activated have been reviewed), control passes from thestep 484′ to a step 492′ where the pointer that keeps track of elementson the list is made to point to the first item in the list of standardlogical device/virtual device pairs. Following the step 492′ is a teststep 494′ where it is determined if the entire list has been processed.If not, then control transfers from the step 494′ to a step 495′ whereexclusive access to appropriate resources (e.g., the standard logicaldevice) is enabled in a manner similar to that discussed above inconnection with FIGS. 15B and 15C. Following the step 495′ is a step496′ which activates the session corresponding to the protection bitthat has been set for the standard logical device/virtual device pair.Activation the session at the step 496′ is analogous to the processingperformed at the step 424 in the flow chart 420, of FIG. 15C, discussedabove. Activating the session at the step 496′ causes the specialvirtual device processing, discussed elsewhere herein, to be performedwhen a write occurs to a track of the standard logical device having aset protection bit. Following the step 496′ is a step 497′ whereexclusive access to appropriate resources is disabled in a mannersimilar to that discussed above. Following the step 497′ is a step 498′where the pointer that is keeping track of the processed items on thelist is made to point to the next item. Following the step 498′, controltransfers back to the test step 494′.

If it is determined at the test step 494′ that the entire list ofstandard logical device/virtual device pairs has been processed (so thatall standard logical devices corresponding to virtual devices beingactivated have had the corresponding sessions made active), controltransfers from the step 494′ to a step 512′ where the list is cleared sothat a subsequent call to the Activate routine will not attempt toreactivate already activated standard logical device/virtual devicepairs. Following the step 512′, processing is complete. Note that, justas with FIGS. 15B and 15C, for some embodiments it may not be necessaryto enable and disable exclusive access to the resources used inconnection with the step 496′. This is illustrated by alternative paths513 a′, 513 b′.

The alternative embodiment illustrated by the flow chart 480′ of FIG.19B may be used in instances where it is not essential that all standardlogical device/virtual device pairs be activated synchronously and/or ininstances where a host application performs processing to synchronizeactivations when necessary. Different options for host applicationprocessing that may be used with FIG. 19B (or FIG. 19A) is discussedelsewhere herein.

In some instances, it may be desirable to restore a virtual device backto the corresponding standard logical device or to another standardlogical device. That is, it may be useful to convert the virtual deviceto an actual logical device with its own data storage or transfer avirtual device to another virtual device.

Referring to FIG. 20A, a standard logical device 532 is shown ascorresponding to a virtual device 534 and a log device 536 where it isdesirable to restore the virtual device 534 to a logical device havingits own storage. Note that, in the example of FIG. 20A, some tracks ofthe standard logical device 532 that are pointed to by the virtualdevice 534 have not changed since the virtual device 534 wasestablished. Other tracks of the log device 536 that are pointed to bythe virtual device 534 correspond to tracks of the standard logicaldevice 532 that have changed since the virtual device 534 wasestablished.

Referring to FIG. 20B, the results of restoring the virtual device 534to the standard logical device 532 of FIG. 20A (the old std dev) areshown. The virtual device 534 has been eliminated. In addition, for anytracks of the standard logical device 532 that are pointed to by thevirtual device 534, no special processing has been performed. However,for tracks of the log device 536 that are pointed to by the virtualdevice 534, the corresponding tracks of the standard logical device 532are modified to point to the corresponding tracks of the log device 536.Thus, a host accessing the standard logical device 532 will, in effect,access the data on the track of the log device 536. This indirectionmechanism provides a way to restore the standard logical device 532without having to immediately move all of the data from the log device536 to the standard logical device 532. In an embodiment herein, theappropriate tracks of a standard logical device 532 are made to point tocorresponding tracks of a log device 536 and, in addition, a backgroundcopy process copies to the standard logical device 532 any tracks thatare pointed to by the standard logical device 532. In an embodimentherein, the background copy process is designed to not appreciablyinterfere with normal access of the standard logical device 532. Inaddition, in the case of accessing a track of the standard logicaldevice 532 that points to a track of the log device 536, a copy is alsoperformed when the track is accessed rather than waiting for thebackground task to copy the track. Note that when a virtual device isrestored to the standard logical device to which the virtual device wasestablished, there are no other virtual devices established to thestandard logical device. However, for other types of restore, discussedbelow, there may be more than one virtual device established to astandard logical device prior to restoring the virtual device to anotherstandard logical device or to another virtual device.

Referring to FIG. 20C, another type of restore is illustrated where thedata represented by the virtual device 534 of FIG. 20A is copied to anew standard logical device 538. In that case, the new standard logicaldevice 538 consists entirely of indirect tracks that point to either thestandard logical device 532 in instances where the virtual device 534previously pointed to the standard logical device 532 or, alternatively,point to the log device 536 in instances where the virtual device 534previously pointed to the log device 536. As in the embodimentillustrated in FIG. 20B, a background copy task may be used to copy datafrom the standard logical device 532 and the log device 536 to thetracks of the standard logical device 538 so that, eventually, thestandard logical device 538 will not contain any indirect tracks fromthe restore operation. Also, as in the embodiment of FIG. 20B, access toa particular track of the standard logical device 538 may cause thattrack to be copied in connection with the access rather than waiting forthe background copy to move the track.

Referring to FIG. 20D, a mirror logical device 532′ is a logical volumethat represents a point in time copy of the standard logical device 532.Like the standard logical device 532, the mirror logical device 532′contains its own storage for tracks of data. In an embodiment herein,the mirror logical device 532′ is first established to the standardlogical device 532, which initially causes data to be copied from thestandard logical device 532 to the mirror logical device 532′. Once theinitial copying is complete, the logical devices 532, 532′ are “synced”.Write operations performed to the standard logical device 532 are alsoperformed to corresponding tracks of the mirror logical device 532′. Anyread operations for a track not in cache may be performed to either ofthe devices 532, 532′.

After the mirror logical device 532′ has been established to thestandard logical device 532, it is possible to “split” the devices 532,532′ so that operations performed on one of the devices 532, 532′ (e.g.,writes) are not automatically performed on the other one of the devices532, 532′. However, even after splitting the devices 532, 532′, theremay be a mechanism that keeps track of the changes that occur after thesplit so that, for example, it is possible to resync the devices 532,532′ after a split without having to copy all of the data from thestandard logical device 532 to the mirror logical device 532′.

In the embodiment illustrated by FIG. 20D, the mirror logical device532′ was split from the standard logical device 532 after the virtualdevice 534 was established to the standard logical device 532. In thiscase, restoring the virtual device 534 to the mirror logical device 532′is like restoring the virtual device to the standard logical device 532as shown in FIG. 20B, where tracks of the mirror logical device 532′that have changed after the virtual device 534 was established are madeto be indirect pointers to tracks of the log device 536 and tracks thatdid not change after the virtual device 534 was established may beaccessed directly on the mirror logical device 532′. This is explainedin more detail below. Note that the embodiment of FIG. 20D exhibits theadvantages of the embodiment of FIG. 20B (less indirection and lessbackground copying) and the advantages of the embodiment of FIG. 20C(standard logical device 532 is not modified to perform the restore).

Referring to FIG. 20E, the virtual device 534 is restored to a newvirtual device 534′. In this case, the new virtual device 534′ isessentially a copy of the original virtual device 534, with pointers tothe standard logical device 532 and the log device 536.

In some embodiments, it may be possible to provide an optional parameterfor restoring that allows changing the name of the device to which thevirtual device is being restored in connection with the restoration.Similarly, an optional parameter may be used so that the device to whichthe virtual device is being restored may be made on line or off line. Inaddition, it may be possible to restore multiple devices synchronouslyusing Begin Group and Process Group commands similar to those discussedabove in connection with establishing virtual devices.

Referring to FIG. 21, a flow chart 550 illustrates steps performed inconnection with performing a restore like that illustrated in FIG. 20Band discussed above where a virtual device is restored to a standardlogical device to which the virtual device was previously established.Processing begins at a first step 552 where the first track of thevirtual device is pointed to. A pointer is used to iterate through andprocess each track of the virtual device. Following step 552 is a teststep 554, which determines if there are more tracks to be processed. Ifnot, then control transfers from the step 554 to a step 555 where avalue indicating that a restore has been performed for the standardlogical device is written to a portion of the header field of the devicetable corresponding to the session that had been used for the virtualdevice. Following the step 555 is a step 557 where the virtual device isdeallocated in a manner similar to that described above in connectionwith FIG. 14, except, of course, that the log tracks are not returned tothe free list.

Following step 557, is a step 558 where the session type is changed. Inan embodiment herein, the session type is changed at the step 558 toindicate that the standard logical device has tracks thereon thatindirectly point to tracks of the log device (explained in more detailbelow). For this session type, tracks are gradually migrated from thelog device to the standard logical device. This migration may beperformed by a system-wide task that resolves indirect tracks for thesession type set at the step 558. Alternatively, it is also possible toexplicitly start a background copy task (like a background copy thatmight be used in a snap operation). The background copy task resolvesindirect pointers by copying data from tracks of the log device to thestandard logical device. Note that, as tracks are moved from thestandard logical device to the log device, tracks of the log device thatare not pointed to by other virtual devices are returned to the freelist in a manner similar to that discussed above in connection with thesteps 386, 388 of FIG. 14. Note also that if a write occurs to anindirect track of the standard logical device that points to a track ofthe log device, the track containing the data is first copied from thelog device to the standard logical device (and possibly returned to thefree list of log device tracks) prior to the write being executed. Inother embodiments, writes to an indirect track where the data is on thelog device may cause the write to be executed directly to the log devicetrack. Following the step 558, processing is complete.

If it is determined that the test step 554 that there are more tracks tobe processed, then control passes from the step 554 to a test step 562where it is determined that if the virtual device track being processedpoints to the standard logical device. If not (meaning that the track ofthe virtual device points to the log device), then control transfersfrom the step 562 to a step 564 where the track of the standard logicaldevice is made to point to the corresponding track of the log device. Inthat way, a subsequent access to the track of the standard logicaldevice will fetch the data stored on the track of the log device.However, as discussed above, the tracks will be migrated from the logdevice to the standard logical device so that, eventually, the standardlogical device will contain the data that is initially provided on thelog device. Note that accessing and modifying tracks of a storage deviceas discussed herein may require inhibiting access by other processes by,for example, locking the tracks prior to determining the state thereofand/or prior to modifying the tracks. Steps for inhibiting access byother processes are not explicitly shown herein, but it is understoodthat such steps will be performed when needed.

Following the step 564 is a step 567 where it is determined if theprotection bit for the track is set. Since the track of the virtualdevice points to the log device, it is expected that the protection bitwould be clear at this step, in accordance with other processingdiscussed herein. Thus, if it is determined at the step 567 that theprotection bit for the track is set, control passes from the step 567 toa step 568, where error processing is performed. The error processingmay include simply logging the error, suspending processing andreturning an error indicator, taking steps to correct the error, etc.

If it is determined at the step 567 that the protection bit for thetrack is not set, or following the step 568, control transfers to a step572 where the next track of the virtual device is pointed to in order toperform the processing described herein. Following the step 572, controltransfers back to the test step 554, discussed above.

If it is determined at the test step 562 that the virtual device trackbeing processed points to the standard logical device, control transfersfrom the step 562 to a step 574, where it is determined if theprotection bit for the track is set. In accordance with other processingdiscussed herein, it is expected that the protection bit would be set ifthe virtual device track (table) points to the standard logical device.Thus, if it is determined at the step 574 that the protection bit is notset, control transfers from the step 574 to the step 568, discussedabove.

If it is determined at the step 574 that the protection bit is set (asexpected), control transfers from the step 574 to a step 578, where theprotection bit is cleared so that no special processing will beperformed on behalf of the previous standard logical device/virtualdevice session in connection a write to the track of the standardlogical device. Note however that, as discussed below, other types ofrestore operations may require special processing in connection withwrites to tracks of the standard logical device. Following the step 578,control transfers to the step 572 to process the next track.

Referring to FIG. 22, a flow chart 580 illustrates steps performed inconnection with restoring a virtual device to a new standard logicaldevice as illustrated in FIG. 20C and discussed above. Processing beginsat a first step 582 where a first track of the virtual device is pointedto. Following step 582 is a test step 583 where it is determined ifthere are more tracks of the virtual device to be processed. If not,then control transfers from the step 583 to a step 584 where a valuethat indicates that a restore has been performed for the virtual deviceis written to a portion of the header field of the device table of thestandard logical device corresponding to the session that had been usedfor the virtual device in a manner similar to that discussed above inconnection with the step 555. Note, however, that in the case of theflow chart 580, there is no corresponding step for clearing all of theprotection bits. This is because, as discussed in more detail below, theprotection bits are used after the restore in this embodiment.

Following the step 584 is a step 585 where the virtual device isdeallocated in a manner similar to that described above in connectionwith FIG. 21. Following the step 585 is a step 586 where the sessiontype is changed to facilitate migrating tracks in a manner similar tothat discussed above in connection with the step 558. Note in this case,however, that all of the tracks of the new standard logical device areindirect, so that tracks will be migrated from both the log device andthe standard logical device that was previously associated with thevirtual device that is being restored. However, just as with theembodiment of FIG. 21, free tracks of the log device will be returned tothe free list and writes to indirect tracks may cause the track to becopied to resolve the indirection. In other embodiments, writes to anindirect track on the log device may cause the write to be executeddirectly to the log device track. Following step 586, processing iscomplete.

If it is determined at the test step 583 that there are more tracks tobe processed, then control transfers from the step 583 to a test step588, where it is determined if the track (table) of the virtual devicepoints to the standard logical device. If so, then control transfers toa test step 589, where it is determined if the protection bit is set forthe track. In accordance with the other processing discussed herein, itis expected that the protection bit would be set of the virtual devicepoints to the standard logical device. Accordingly, if it determined atthe step 589 that the protection bit is not set, then control transfersfrom the step 589 to a step 591 where an error processing is provided ina manner similar to that discussed above in connection with the step568. In some embodiments, the processing at the step 591 includessetting the protection bit to the correct value. If it is determined atthe step 589 that the protection bit for the standard logical device isset, or following the step 591, control transfers to a step 592, wherethe track of the new standard logical device is set to be an indirectpointer to the track of the old standard logical device (i.e., thestandard logical device previously established to the virtual devicebeing restored).

If it is determined at the test step 588, that the track (table) of thevirtual device does not point to the standard logical device, controltransfers to a test step 593, where it is determined if the protectionbit is set for the track. In accordance with the other processingdiscussed herein, it is expected that the protection bit would not beset if the virtual device does not point to the standard logical device(i.e., points to the log device). Accordingly, if it determined at thestep 593 that the protection bit is set, then control transfers from thestep 593 to the step 591, discussed above. If it is determined at thestep 593 that the protection bit for the standard logical device is notset, or following the step 591, control transfers to a step 595, wherethe track of the new standard logical device is set to be an indirectpointer to the corresponding track of the log device.

Following the step 592 or the step 595 is a step 598 to process the nexttrack of the virtual device. Following the step 598, control transferback to the step 583, discussed above.

As mentioned above, the protection bits corresponding to the restoredvirtual device session are not cleared in the embodiment illustrated byFIG. 22. This may be explained with reference to FIGS. 20A and 20C.First, note that some of the tracks of the standard logical device 532may never have been written to after the virtual device 534 wasestablished, and thus those tracks remain in their original state on thestandard logical device 532. When the virtual device 534 is firstrestored to the new standard logical device 538 as shown in FIG. 20C,those tracks of the standard logical device 532 are indirectly pointedto by corresponding tracks of the standard logical device 538. If asubsequent write to one of those tracks on the standard logical device532 were to occur prior to the track being copied to the standardlogical device 538, then the standard logical device 538, with anindirect reference to the newly written track on the standard logicaldevice 532, would no longer correspond to a restored version of thevirtual device 534. However, the set protection bit for the trackprevents this. After a restore such as that illustrated in FIG. 20C isperformed, a write to a track of the standard logical device 532 havinga set protection bit causes the track to first be copied to the standardlogical device 538 before the write occurs. Note that, the protectionbit for each track may be cleared after each track is copied from thestandard logical device 532 to the standard logical device 538,irrespective of whether the copy occurred in connection with the specialprocess caused by the set protection bit or by track migration initiatedat the step 588. Thus, once all of the data has been migrated from thestandard logical device 532 to the standard logical device 538, all ofthe protection bits associated with the session are expected to beclear.

Referring to FIG. 23, a partial flow chart 600 illustrates stepsperformed to restore a virtual device to a mirror logical device, asillustrated above in connection with FIG. 20D. The flow chart 600illustrates steps performed that are different from those of the flowchart 550 of FIG. 21. Portions of the flow chart 600 which interface andflow into the flow chart 550 (i.e., where the same operations areperformed) are shown in FIG. 23 and described herein.

Processing begins at a first step 602 where all changes (e.g., writes)performed on the mirror logical device since the split operationoccurred are undone, rendering the mirror logical device substantiallyidentical to the standard logical device at a point in time just priorto the split. For embodiments disclosed herein, a split may occur afterthe virtual device is established to the standard logical device butbefore the virtual device is restored. The ability to undo the changessince the split may be provided by the mirror facility that uses featuretracking to log and track the changes that occurred since the split.

Following the step 602 is a step 604, which is like the step 552 of FIG.21 where a pointer is set to point to the first track of the virtualdevice. Following the step 604 is a test step 606, which is like thetest step 554 of FIG. 21, where it is determined if there are moretracks to be processed. If not, then control transfers to the step 555of FIG. 21, discussed above. Otherwise, control transfers to a test step608, which is like the step 562 of FIG. 21, where it is determined ifthe pointer of the virtual device points to the standard logical device.If so, then control transfers to the step 574 of FIG. 21, discussedabove.

If it is determined at the step 608 that the track of the virtual devicebeing processed does not point to the standard logical device(indicating at least one write was performed after the virtual devicewas established), then control transfers from the step 608 to a step612, where it is determined if all of the write operations to the trackbeing processed occurred after the mirror logical device was split fromthe standard logical device. Note that if this is the case, the undooperation at the step 602 will have restored the track of the mirrorlogical device to the same state as the corresponding track of thestandard logical device prior to the virtual device being established.

If it is determined that all writes to the track did not occur after thesplit, then control transfers from the step 608 to the step 564 of FIG.21, discussed above, to cause the corresponding track of the mirrorlogical device to indirectly point to the appropriate track of the logdevice in a manner similar to that discussed above in connection withFIG. 21. If it is determined at the test step 612 that all writesoccurred after the split, then control transfers from the step 612 to atest step 614, where it is determined if the virtual device is the lastvirtual device (i.e., the only virtual device) pointing to thecorresponding track of the log device. If so, then control transfersfrom the step 614 to a step 616 where the track of the log device isreturned to the free list of the log device tracks. The steps 614, 616are analogous to the steps 386, 388 of FIG. 14, discussed above.Following the step 616, or following the step 614 if the track of thelog device is being used by more than one virtual device, is the step567 of FIG. 21, which tests if the protection bit is in the proper stateand then clears the protection bit. In this instance, however, theprotection bit on both the standard logical device 532 and the mirrorlogical device 532′ are cleared. Note that transferring to the step 567causes the track of the mirror logical device to be a direct track foraccessing data.

It is useful for host applications that access the storage device to beable to use some of the functionality described herein. For example, ahost application may want to create a virtual device to represent apoint in time copy of a standard logical device and then run a backupfrom the virtual device, after which, the virtual device may bedeallocated. In other instances, the virtual device may be used tomaintain a point in time copy of the data from the standard logicaldevice when software that uses the standard logical device is tested.After the testing period, the point in time copy represented by thevirtual device may be restored back in the standard logical device.

Referring to FIG. 24, a diagram 620 illustrates a plurality of hosts622-624 that access the storage device 626. Each of the hosts 622-624may create, establish, deallocate, and restore standard logicaldevice/virtual device pairs as described herein by making system callsto the storage device 626. In an embodiment disclosed herein,applications running on the hosts 622-624 would not directly make thesystem calls. Rather, an underlying layer of software translates higherlevel calls from the hosts 622-624 into the appropriate system callsthat are provided to the storage device 626. For example, the host 622may have an application that establishes a standard logicaldevice/virtual device pair by calling an Establish routine that willultimately cause the appropriate system calls (e.g., Register, Relate,and Activate) to be called using, for example, a library linked to theapplication running on the host 622, an operating system routine thatruns on the host 622, or some other appropriate mechanism.

The parameters passed to the Establish routine may include one or morestandard logical device/virtual device pairs as well as one or moreoptional name change parameters (discussed above) and one or moreonline/offline indicators that determine whether a newly-establishedvirtual device will be on line or off line as discussed above. In someembodiments, an Establish routine called from an application will onlyaccept one standard logical device/virtual device pair. In otherembodiments, an Establish routine will accept multiple standard logicaldevice/virtual device pairs (e.g., a list or list id as discussedabove). In embodiments that accept multiple standard logicaldevice/virtual device pairs, the underlying system calls may or may notcause the virtual devices to be established synchronously in the mannerdiscussed elsewhere herein. In some cases, it may be possible for a hostto handle synchronously establish multiple standard logicaldevice/virtual device pairs by, for example, using the mechanismdisclosed in U.S. patent application Ser. No. 10/134,420 U.S. Pat. No.6,983,353 filed on Apr. 29, 2002, which is incorporated herein byreference, to establish appropriate consistency groups to causesynchronization of the activate operation. In addition, it may bepossible to synchronously establish a plurality of pairs by the hostsimply waiting for all pairs to be established before performingoperations on any of the devices that are part of the pairs. In someembodiments, the Establish routine may be passed one or more standardlogical devices and corresponding virtual devices that are createdand/or obtained by the establish routine.

Referring to FIG. 25, a flow chart 630 illustrates steps performed by anEstablish routine that translates a higher level application call forestablishing one or more standard logical device/virtual device pairsinto appropriate system calls that can be made to the storage device626. The translation may be performed in library code linked to theapplication itself, by the operating system on one of the host devices622-624, or by using any other appropriate mechanism that translateshigh level host application calls to system calls.

Processing begins at a first step 632 where it is determined if morethan one standard logical device/virtual device pair is beingestablished. As discussed above, it is possible in some embodiments toestablish more then one standard logical device/virtual device pair in asingle call and, in some embodiments, in a way that causes all of thevirtual devices to be activated synchronously. If it is determined atthe test step 632 that more than one standard logical device/virtualdevice pair is being established, then control passes from the step 632to a step 634 where a Begin Group call is performed to create the listof standard logical device/virtual device pairs discussed above. Notethat it is possible to synchronously process and establish multiplestandard logical device/virtual device pairs without using Begin Groupand Process Group, as described above. In those cases, anotherappropriate mechanism, such as one or more of those discussed above(e.g., lists, list ids), may be invoked at the step 634 to causemultiple standard logical device/virtual device pairs to be establishedsimultaneously. Alternatively still, it may be possible to establishappropriate consistency groups as described in U.S. patent applicationSer. No. 10/134,420 U.S. Pat. No. 6,983,353, referenced above and usethe consistency group mechanisms to activate multiple pairssynchronously.

If it is determined at the step 632 that only one standard logicaldevice/virtual device pair is to be established, or following the step634, is step 636 where the first standard logical device/virtual devicepair is pointed to in order to facilitate processing thereof. In someembodiments, each standard logical device/virtual device pair is storedin a data structure (containing possibly other optional parameters, suchas a new name and/or an online/offline specifier). The data structuresmay be linked together in a linked list. Thus, the pointer set at thestep 636 and used for follow on processing is provided to traverse thelist. In embodiments where the Establish routine creates or otherwiseprovides corresponding virtual device(s), data location(s) may be passedto the Establish routine, which places one or more appropriate virtualdevice identifiers in the location(s).

Following step 636 is the step 638 which determines if processing of thelist of passed parameters is complete (e.g., if the pointer points tothe end of the list). Of course, on the first iteration, the result ofthe test of the step 638 are expected to indicate that processing is notcomplete. If it is determined at the test step 638 that not all of thestandard logical device/virtual device pairs (or, in some embodiments,just standard logical devices) have been processed, then controltransfer from the step 638 to a step 642 to invoke the Register routine,discussed above.

Following the step 642 is a test step 644 where it is determined if anoptional new name has been specified. If it is determined at the teststep 644 that an optional new name has not been provided, then controltransfers from the step 644 to a test step 646 which determines if anoptional online/offline boolean parameter has been provided. Asdiscussed above, the online/offline option allows the calling routine todetermine whether the virtual device that is being established will beonline (become available to the host) or offline (not available to thehost) upon being established. If it is determined at the test step 646that an online/offline parameters is not being provided, then controlpasses from the test step 646 to a step 648 where a Relate system callis provided to relate the standard logical device/virtual device pair.

If it is determined at the test step 646 that an optional online/offlineparameter has been provided, then control transfers from the step 646 toa step 652 where the Relate system call is made. However, at the step652, the online/offline parameters may also be passed by the system callto indicate whether the virtual device should be made online or offlineat the time of establishment.

If it is determined at the test step 644 that a new name for the virtualdevice has been provided, then control transfers from the step 644 to astep 653 where it is determined if an optional online/offline parameterhas been provided. If it is determined at the test step 655 that anoptional online/offline parameters is not being provided, then controltransfers from the step 653 to a step 654 to provide a system call toRelate the pair along with the new name parameter, as discussed above.

If it is determined at the test step 653 that an optional online/offlineparameter has been provided in connection with the standard logicaldevice/virtual device pair, then control transfers from the step 653 toa step 656 where the pair is related along with the new name for thevirtual device and with an indication of whether the virtual device willbe online or offline upon establishment.

Following each of the steps 648, 652, 654, 656, is a step 658 where thepointer set at the step 636 is made to point to the next standardlogical device/virtual device pair, if any. Following step 658, controltransfers back to the test up 638 to determine if all of the standardlogical device/virtual device pairs have been processed.

If it is determined at the test step 638 that all the standard logicaldevices/virtual device pairs have been processed, than control transfersfrom the step 638 to a step 662 where an Activate system call isprovided for each of the pairs (or, for different embodiments discussedabove, Activate may be called for all of the pairs). Following step 662is a step 664 where it is determined if there is more than one standardlogical device/virtual device pair. If so, then controlled transfersfrom the step 664 to a step 666 where a Process Group call is made. Justas with the step 634 where Begin Group has been called, the step 666 mayrepresent another mechanism to synchronously establish multiple standardlogical device/virtual device pairs. In some embodiments, the routineperforming the processing illustrated by the flow chart 630 keeps trackof a list of standard logical device/virtual device pairs, in which casethe entire list (or a list id, as discussed above) may be passed to theActivate routine, as discussed above. In other embodiments, aconsistency group is formed, as described in U.S. patent applicationSer. No. 10/134,420 U.S. Pat. No. 6,983,353, mentioned above.

Following the step 666 or following the step 664 if there is only onestandard logical device/virtual device pair, is a step 668 where theresult of performing the processing set forth in the previous steps isreturned to the calling routine (e.g., success or failure and, in someembodiments, identifiers for the newly established virtual devices).Following the step 668, processing is complete.

It is also possible to provide a Restore routine that is called from anapplication at one of the hosts 622-624 where the Restore routine takes,as parameters, one or more standard logical device/virtual device pairsas well as optional names for renaming the standard logical device(s) towhich the virtual devices(s) are restored and optional online/offlineboolean values indicating whether the standard logical device(s) will beonline or offline. The parameters may be passed in a linked list in amanner similar to that discussed above in connection with the Establishroutine. Note also that, in the case of the Restore routine, thestandard logical device of a standard logical device/virtual device pairmay be the same standard logical device to which the virtual device wasestablished or could be a different standard logical device or a mirrorlogical device, as discussed above. Also as discussed above, a virtualdevice may be restored to another virtual device.

Referring to FIG. 26, a flow chart 680 illustrates steps performed inconnection with a host application calling a Restore routine to restorea virtual device as illustrated in FIGS. 20B, 20C, 20D, 20E. Processingbegins at a first step 692 where it is determined if an optional newname has been provided as a parameter to the Restore routine. Asdiscussed above, in some instances it is possible to restore a virtualdevice to a standard logical device or to a new virtual device and, atthe same time, provide the standard logical device or new virtual devicewith a different name than that associated with the old virtual device.If it is determined at the test step 692 that a new name has not beenprovided, then control transfers from the step 692 to a test step 694where it is determined if an optional online/offline parameter has beenprovided. As discussed above, in some instances it is possible torestore a virtual device to a standard logical device or new virtualdevice while, at the same time, making the standard logical device ornew virtual device not available to the host (offline).

If it is determined at the test step 694 that an optional online/offlineparameter has not been provided, then control transfers from the step694 to a step 696 where the virtual device is restored by making aRestore system call, as discussed above. Alternatively, if it isdetermined at the test step 694 that an optional online/offlineparameter has been provided, then control transfers from the test step694 to a step 698 where a Restore system call is provided with theoptional online/offline parameter.

If it is determined at the test step 692 that a new name has beenprovided to the Restore routine, then control transfers from the step692 to a test step 702 where it is determined if an optionalonline/offline parameter has also been provided. If not, then controltransfers from the test step 702 to a step 704 where a Restore systemcall is provided, along with the new name of the standard logical deviceor new virtual device, to restore the virtual device. If it isdetermined that the test step 702 that an optional online/offlineparameter has been provided, then control transfers from the test step702 to a step 706 where a Restore system call is performed with the newname and online/offline parameter. Following any of the steps 696, 698,704, 706, processing is complete.

In some instances, it may be desirable to deactivate (rather thandeallocate) a virtual device such that, while the virtual device is nolonger accessible, the tracks written to the log device in connectionwith using the virtual device are still available. That is, a call todeactivate a virtual device makes the virtual device not ready to anyhost that attempts to access it and causes writes to the correspondingstandard logical device to not result in any special processing such asthat discussed above. In effect, a deactivated virtual device representsthe tracks of the corresponding standard logical device that changedfrom the time that the virtual device was established until the timethat the virtual device was deactivated. For example, if a virtualdevice was established at noon and deactivated at 6 p.m., then thetracks of the log device pointed to by the table of the deactivatedvirtual device represent only those tracks of the standard logicaldevice that changed between noon and 6 p.m.

Referring to FIG. 27, a flow chart 730 illustrates steps performed inconnection with deactivating a virtual device. Processing begins at afirst step 732 where exclusive access to appropriate resources (e.g.,the virtual device) is enabled in a manner similar to that discussedabove in connection with FIGS. 15B and 15C. Following the step 732 is astep 734 where exclusive access to appropriate resources (e.g., thecorresponding standard logical device) is enabled in a manner similar tothat discussed above in connection with FIGS. 15B and 15C.

Following the step 734 is a step 736 where the session corresponding tothe standard logical/virtual device pair is made inactive by, forexample, writing an appropriate value to the device header field.Following this, any writes to the standard logical device will not causeany special processing to occur. This effectively freezes the changes tothe virtual device.

Following the step 736 is a step 738 where exclusive access toappropriate resources of the standard logical device is disabled in amanner similar to that discussed above. Note that since the session ismade inactive at the step 736, any subsequent writes to the standardlogical device will not effect the virtual device. Following the step738 is a step 742 where the virtual device is made not ready. Making thevirtual device not ready at the step 742 will prevent the virtual devicefrom being modified by, for example, a host. However, the tablecorresponding to the virtual device is maintained in order to provideaccess to the tracks of the log device corresponding to the virtualdevice. In other embodiments, it is possible to read the tracks of thelog device through the virtual device (i.e., read the virtual device) bymaking the virtual device not ready for writes only at the step 742 inconjunction with making the session inactive at the step 736.

Following the step 742 is a step 744 where exclusive access toappropriate resources of the standard logical device is disabled in amanner similar to that discussed above. Note, however, that theprocessing at the step 744 does not facilitate access (or at least writeaccess) by any host device to the virtual device, since the virtualdevice was made not ready at the step 742. In addition, the processingat the step 744 does not cause data from the corresponding standardlogical device to be written to the virtual device since, as describedabove, the session was made inactive at the step 736 in order to preventfurther manipulation of the virtual device on account of the standardlogical device. Following the step 744, processing is complete.

Note that, just as with FIGS. 15B and 15C, for some embodiments it maynot be necessary to enable and disable exclusive access to the resourcesused in connection with one or more of the steps 736, 742. This isillustrated by alternative paths 746 a-746 d.

A host application may access the tracks of the log device correspondingto the deactivated virtual device by making two system calls. The firstsystem call reads the protection bits for the standard logical device,and thus provides information as to which tracks of the virtual devicepoint to the log device, and thus correspond to tracks modified inbetween the time that the virtual device is established and the virtualdevice is deactivated. That is, for any protection bit of the standardlogical device that is set, there is no corresponding track of thedeactivated virtual device that points to the log device. Conversely, ifa bit for a track of the standard logical device is clear, thusindicating that a write to the track occurred after the virtual devicewas established but before the virtual device was deactivated, thenthere is a corresponding track on the log device pointed to by thedeactivated virtual device. This system call would pass as a parameteran identifier for a track and would receive back the state of theprotection bit for the track. Alternatively, the system call could bepassed a pointer to a data structure for storing the state of a range oftracks (including all the tracks) that are filled in and returned to thecalling routine.

Another system call would allow reading of saved tracks corresponding toa deactivated virtual device. The routine for reading of save tracks ofa deactivated virtual device would take, as parameters, an identifierfor the virtual device and a track identifier indicating which track isto be read. Note that one possible mechanism for reading log trackscorresponding to a deactivated virtual device is to have the virtualdevice be not ready for writing only and to deactivate the sessioncorresponding to the virtual device. In such a case, the log tracks maybe accessed through the virtual device by reading the virtual device.

Referring to FIG. 28, a flow chart 750 illustrates steps performed in anembodiment that reads a track of a deactivated virtual device.Processing begins at a first step 752 where it is determined if thevirtual device passed as a parameter to the routine for reading trackscorresponding to deactivated virtual devices is in fact a deactivatedvirtual device. If not, then control passes from the step 752 to a step754 where an error is returned to the calling routine. Following thestep 754, processing is complete.

If it is determined at the step 752 that the virtual device identifierthat is passed by the calling routine corresponds to a deactivatedvirtual device, then control passes from the step 752 to a step 756where it is determined if the requested track corresponds to a track onthe virtual device that points to the log device. As discussed above,only some of the tracks of the virtual device will correspond to a trackof the log device (i.e., tracks that correspond to tracks of thestandard logical device that were written after the virtual device wasestablished and before the virtual device is deactivated). Tracks whichhave not been written to in between the time that the virtual device isestablished and the virtual device is deactivated will not be pointed toon the log device by the virtual device. Thus, if it is determined atthe step 756 that the track identifier passed by the calling routinedoes not correspond to a track pointed to by the virtual device, thencontrol passes from the step 756 to the step 754 where an error isreturned, as discussed above. Otherwise, if it is determined at the step756 that the track is pointed to by the virtual device, then controlpasses from the step 756 to a step 758 where the requested track data isreturned. Following the step 758, processing is complete. Note that, forsome embodiments, it may be possible to request data from a plurality oftracks with one call.

At some point, it may be useful to deallocate a deactivated virtualdevice. In that case, it would be possible to invoke the routine whichis discussed above in connection with the flow chart 370 of FIG. 14.Note that the steps shown in the flow chart 370 are the same as thosethat would be executed to deallocate a deactivated virtual device.

In some instances, it is useful to be able to obtain data that existedon a computer system at a particular time in the past. In systems wheredaily backups are made and saved, it may be possible to request a fileor a set of files from a particular day. However, in some cases, thegranularity of one day is not sufficient for certain applications. Inaddition, such a system may require saving an entire system's worth ofdata every day, which may be considerable. In other cases, it may onlybe necessary to provide incremental backups. However, incrementalbackups typically require that the data be first backed up completelyand then, going forward, only files that have changed since the completebackup or the most recent incremental backup are saved. A difficultywith this is that entire files are saved even though, in some cases,only one track of a file may have changed. In addition, retrieving afile from a particular day may require restoring the entire file andthen rolling the system forward. It would be desirable in some casesinstead to be able to start with the current state of the system and beable to work backwards.

Referring to FIG. 29, a flow chart 760 illustrates a backup system thatuses virtual devices to provide track level incremental backups. Thesteps of the flow chart 760 may be performed by a host system running ahost application. Processing begins at a first step 762 where a newvirtual device is established to a standard logical device that is thesubject of the backup. Following the step 762 is a step 764 where an oldvirtual device that had been previously established to the standardlogical is deactivated. Following the step 764 is a step 766 where thetracks corresponding to the old virtual device are saved. Saving thetracks may involve collecting the tracks pointed to by the deactivatedvirtual device (i.e., tracks that were modified in between the time thatthe virtual device was established and the time that the virtual devicewas deactivated) using, for example, the mechanism discussed above. Thetracks may be saved by indicating a corresponding source track of thestandard logical device (which, as discussed above, is saved with eachtrack of the log device). Other information that may be saved mayinclude a time stamp or an identifier indicating which backup sessionthe saved tracks are from.

The steps of the flow chart 760 may be run every incremental period suchas every six hours. Thus, a user wishing to have a version of a filefrom one of those six hour increments would be able to do so, asdiscussed below. Note also that it is possible in some embodiments todeallocate the deactivated virtual device after executing the step 766.

Referring to FIG. 30, a flow chart 780 illustrates steps performed inconnection with restoring data using the tracks saved in connection withthe flow chart 760 of FIG. 29. The steps of the flow chart 780 may beperformed by a host system running a host application. Processing beginsat a first step 782 which starts with the current data image. Followingstep 782 is a step 784 where a pointer is set to point to the mostrecent tracks that have been saved. Following step 784 is a step 786where the tracks that are pointed to are applied to the image. The step786 represents copying saved tracks to the image to roll the image backto its state at an earlier time.

Following the step 786 is a step 788 where the pointer is made to pointto the next most recent set of saved tracks. Following the step 788 is atest step 792 where it is determined if processing is complete (i.e., ifthe rolling backup has worked backwards to the desired time). If it isdetermined at the step 792 that processing is not complete, then controltransfers back to the step 786. Otherwise, it may be determined at thestep 792 that processing is complete.

While the invention has been disclosed in connection with variousembodiments, modifications thereon will be readily apparent to thoseskilled in the art. Accordingly, the spirit and scope of the inventionis set forth in the following claims.

1. A computer implemented method of handling data, comprising: providinga first storage area of a first type that contains sections of data;copying sections of data from the first storage area to a second storagearea of the first type, wherein the second storage area is separate fromthe first storage area; providing a third storage area of a second typewherein the second type has, for each section thereof, a pointer to oneof: a corresponding section of data of the first storage area and acorresponding section of data of the second storage area and wherein thethird storage area is separate from the first storage area and thesecond storage area; causing the third storage area to be not availablefor accessing; and after causing the third storage area to not beavailable for accessing, providing data from the second storage areacorresponding to pointers of the third storage area that point tosections of the second storage area.
 2. A method, according to claim 1,further comprising: after providing the storage areas and prior tocausing the third storage area to not be available, handling a write toa particular section of the first storage area pointed to by acorresponding pointer of the third storage area by copying data from theparticular section of the first storage area to a corresponding sectionof the second storage area and adjusting the corresponding pointer ofthe third storage area to point to the corresponding section of thesecond storage area.
 3. A method, according to claim 2, furthercomprising: after causing the third storage area to not be available,handling a write to a particular section of the first storage area bywriting the data thereto.
 4. A method, according to claim 2, whereincausing the third storage area to not be available includes providing avalue in a header for the first storage area, wherein the valueindicates that no operation is to be performed in connection with a setprotection bit encountered when data is written to a correspondingsection of the first storage area.
 5. A method, according to claim 4,further comprising: inhibiting access to the first storage area prior toproviding the value to the header; and allowing access to the firststorage area after providing the value to the header.
 6. A method,according to claim 1, wherein the storage areas are devices.
 7. Acomputer implemented method of retrieving requested data from a virtualstorage area, comprising: determining if the virtual storage area isdeactivated, wherein the virtual storage area contains pointers tosections of data of a logical storage area and wherein, in response to awrite to a section of the logical storage area, data is copied from thelogical storage area to an other area and the virtual storage area isadjusted to point to the other area; if the virtual storage area is notdeactivated, then providing the requested data using the virtual storagearea; and if the virtual storage area is deactivated and the requesteddata corresponds to data handled by the virtual storage area prior tobeing deactivated, then providing the requested data from the otherarea.
 8. A method, according to claim 7, wherein determining if therequested data corresponds to data handled by the virtual storage areaprior to being deactivated includes examining protection bits of acorresponding standard logical storage area.
 9. A method, according toclaim 8, wherein the storage areas are devices.
 10. A method, accordingto claim 7, wherein providing the requested data includes reading thevirtual storage area.
 11. A computer implemented method of restoringdata to a previous version, comprising: providing a virtual storage areacontaining pointers to sections of data of a logical storage area; inresponse to a write to a section of the logical storage area, copyingdata from the logical storage area to an other area containing archivedsections of data and causing the virtual storage area to point to theother area; obtaining a current version of the data; obtainingpreviously archived sections of the data; and iteratively applying thepreviously archived sections of data to the current version and toresulting intermediate versions until the data corresponds to theprevious version of the data.
 12. A method, according to claim 11,wherein previously archived sections correspond to versions of thesections that existed prior to archiving.
 13. Computer software,provided in a computer readable storage medium, the computer softwarecomprising executable code that causes a computing device or processorto handle data used in connection with a first storage area of a firsttype that contains sections of data, a second storage area of the firsttype, wherein the second storage area is separate from the first storagearea, and a third storage area of a second type wherein the second typehas, for each section thereof, a pointer to one of: a correspondingsection of data of the first storage area and a corresponding section ofdata of the second storage area and wherein the third storage area isseparate from the first storage area and the second storage area, thesoftware comprising executable code that copies sections of data fromthe first storage area to the second storage area; executable code thatcauses the third storage area to be not available for accessing; andexecutable code that, after the third storage area is not available foraccessing, provides data from the second storage area corresponding topointers of the third storage area that point to sections of the secondstorage area.
 14. Computer software, according to claim 13, whereinexecutable code that causes the third storage area to not be availableincludes executable code that provides a value in a header for the firststorage area, wherein the value indicates that no operation is to beperformed in connection with a set protection bit encountered when datais written to a corresponding section of the first storage area. 15.Computer software, provided in a computer readable storage medium, thecomputer software comprising executable code that causes a computingdevice or processor to retrieve requested data from a virtual storagearea, the software comprising: executable code that determines if thevirtual storage area is deactivated, wherein the virtual storage areacontains pointers to sections of data of a logical storage area andwherein, in response to a write to a section of the logical storagearea, data is copied from the logical storage area to an other area andthe virtual storage area is adjusted to point to the other area;executable code that, if the virtual storage area is deactivated, thendetermines if the requested data corresponds to data handled by thevirtual storage area prior to being deactivated; and executable codethat, if the requested data corresponds to data handled by the virtualstorage area prior to being deactivated, then provides the requesteddata from the other area.
 16. Computer software, according to claim 15,further comprising: executable code that examines protection bits of acorresponding standard logical storage area.
 17. Computer software,according to claim 15, further comprising: executable code that readsthe virtual storage area.
 18. Computer software, provided in a computerreadable storage medium, the computer software comprising executablecode that causes a computing device or processor to restore data to aprevious version using a virtual storage area that contains pointers tosections of a logical storage area, the software comprising: executablecode that copies data from the logical storage area to an other areathat contains archived sections of data and causes the virtual storagearea to point to the other area in response to a write to a section ofthe logical storage area; executable code that obtains a current versionof the data; executable code that obtains previously archived sectionsof the data; and executable code that iteratively applies the previouslyarchived sections of data to the current version and to resultingintermediate versions until the data corresponds to the previous versionof the data.
 19. Computer software, according to claim 18, whereinpreviously archived sections correspond to versions of the sections thatexisted prior to archiving.